Mail Server

The Synchronet Mail Server is responsible for incoming and outgoing Internet e-mail.

The mail server accepts submissions via the SMTP and ESMTP protocols, by default, on TCP ports 25 and 587.

By default, the mail server does not allow relaying of mail from unauthenticated clients to external mail servers.

The SMTP server supports authentication via the PLAIN, LOGIN, or CRAM-MD5 SMTP-AUTH methods (client chooses).

The SMTP server optionally supports SMTP-TLS sessions.

The SMTP server can service multiple simultaneous incoming sessions and messages, with multiple recipients per message.

The SMTP server also supports the SEND, SAML and SOML commands for delivering instant messages (a.k.a. telegrams) to users of the BBS.

When receiving email messages via SMTP, the Synchronet mail server can match recipient names to a BBS user account by their alias or optionally, their real name.

Non-alphanumeric characters are ignored when matching recipient names to BBS user aliases. So for example, the following will all match the user “digital man”:

• digital.man
• digital_man
• digitalman
• d-i-g-i-t-a-l-man

When matching recipient names to BBS real names (if so enabled by the sysop by setting SCFG->Message Options->Receive E-mail by Real Name to “Yes”), spaces and '.' are treated as equivalent. When “Receive E-mail by Real Name” is enabled, receiving e-mail by user's alias is still supported.

Recipient names configured in ctrl/alias.cfg can be used to match recipient names to specific user account numbers, aliases, external email addresses, or even sub-boards.

Receiving by user account number is also optionally supported, not not encouraged and disabled by default.

Recipient mail addresses can have special prefixes to direct the routing of the incoming mail:

It is sometimes preferred to use an alias (configured in the ctrl/alias.cfg file) to allow the reception of a more “normal” looking email address and route to the Specially-Prefixed address. The following examples allows the reception of email sent to “cnnreport@<yourbbs>” and forwards the message to the message sub-board with the internal code of dove-deb:

cnnreport    sub:dove-deb

Some specially formatted destination addresses (RCPT TO addresses) are recognized and treated specially by the Synchronet SMTP server.

If the system supports FidoNet-style networking (has at least one address configured in SCFG->Networks->FidoNet->Addresses), then incoming FidoNet netmail messages are recognized with one of the following formats:

• <name>@f<node>.n<net>.z<zone>.fidonet
• <name>@p<point>.f<node>.n<net>.z<zone>.fidonet

Example: rob.swindell@f705.n103.z1.fidonet

Note: the .fidonet host suffix is required even if the destination network is an othernet (not technically part of FidoNet proper). This is a “virtual” TLD, no DNS lookup is performed by the mailserver for destination hostnames with this suffix.

The Synchronet Mail Server recognizes the incoming special address format: <name>#<tag>@<host>

The <tag> is extracted from the address and if it is not found in the data/user/<usernum>.smtpblock file, the mail message is sent to the destination user. If the tag matches a line in the .smtpblock file, the email is rejected by the mail server with a “no such user” error.

This feature allows you (the sysop) to easily create and use “temporary” email addresses without requiring additional user accounts. Once you start receiving spam with a tag, just disable that tag by adding it to your .smtpblock file.

The Synchronet SMTP Server contains several anti-SPAM measures:

• DNS-based blacklist (DNSBL) look-up support with exemptions and multiple possible actions upon positive results
• Mail server filtering based on IP address/subnet or hostname (ctrl/spamblock.cfg), with exemptions (ctrl/spamblock_exempt.cfg)
• Email address filtering with wildcard support (text/email.can)
• Message subject filtering with wildcard support (text/subject.can)
• Configurable SPAM bait list (ctrl/spambait.cfg) for auto-blocking mail servers based on honey-pot destination email addresses
• Unlimited number of external mail processors (e.g. spamc.js, mailauth.js) that can modify (e.g. tag) or reject email messages at will
• Synchronet's normal host filtering (via ip.can and host.can) and configurable LoginAttempt tracking/throttling/blocking (in sbbs.ini) is also employed

DNS-based Blacklists (DNSBL) are 3rd party databases of IP addresses whose hosts are suspected of being habitual spammers or have other security problems which make mail received from these systems highly suspicious. Different DNSBL services/servers have differing criteria for what can get an IP listed or de-listed (removed) from their database. Examine your ctrl/dns_blacklist.cfg file to determine what DNSBL services your Synchronet Mail Server is using.

The Synchronet Mail Server can use DNS queries to multiple DNSBL servers (as configured in ctrl/dns_blacklist.cfg) for each inbound SMTP/ESMTP connection from a prospective mail transfer agent (MTA). If the MTA's IP address is listed by one of the DNSBL servers, then multiple actions may be taken:

• A Notice-level mail server log entry: SMTP BLACKLISTED SERVER on ... (always)
• An entry added to the data/spam.log file (always)
• Mail session refused, if the DNSBL_REFUSE option is set in the Mail section of the sbbs.ini file
• SMTP commands and lines of message headers and body text may be throttled at 1 line per second when the DNSBL_THROTTLE option is set
• Reject the mail message, if the DNSBL_BADUSER option is set
• Message Subject may be 'tagged' with a prepended configurable string (e.g. SPAM:), see DNSBlacklistSubject sbbs.ini key
• Message Header may be 'tagged' with a special header field (e.g. X-DNSBL), see DNSBlacklistHeader sbbs.ini key
• Message may be received and ignored/dropped, if the DNSBL_IGNORE option is set
• Message hash stored in a database of SPAM message body hashes (data/spam.hash), if the DNSBL_SPAMHASH option is set

A mail message's “Received” headers may also be scanned for DNSBL-listed IP addresses (DNSBL_CHKRECVHDRS option) so that any e-mail messages that originate or pass-through a DNSBL-listed will be treated as though it is being delivered directly from a DNSBL-listed MTA.

When a DNSBL service reports an IP address as “black-listed”, any additional DNSBL services/servers are not queried.

MTA's whose IP address or hostname is listed in your ctrl/dnsbl_exempt.cfg file are never checked against DNSBL servers.

Mail from e-mail addresses that are listed in the dnsbl_exempt.cfg file are also exempt from DNSBL actions.

The destination addersses of email delivered by the Synchronet Mail Server's SendMail Thread are automatically added to your dnsbl_exempt.cfg file unless the NO_AUTO_EXEMPT option is set.

Most of the DNSBL-related settings can be found in the sbbsctrl Mail Server Configuration tabs:

In-bound e-mail (received via SMTP) can be intercepted and modified via one or more external mail processors.

An external mail processor needs to create, remove or modify one or more files to have its impact on the SMTP mail receive process. These files (created in the mail server's temporary file directory) are:

Since the file names contain SMTP-session specific information (e.g. numbers), the filenames must be passed from the Mail Server to external mail processors via command-line or JavaScript properties.

An external mail processor may either modify the message_text file in-place or create a new_message_text will be used in its place.

SMTP envelop information is stored in the recipient_list file.

The recipient list file has a .ini [section] for each recipient. Though the section name is an increasing decimal integer value (0, 1, etc.), there's no strict requirement that the recipient sections are so named, as long as they are uniquely named.

An external mail processor may add, remove, or modify recipients in the recipient list file.

An example recipient list file for 4 recipients (3 remote/forward, 1 local):

[0]
To=jack.ryan@vert.synchro.net
ToExt=636
ToNetType=5
ToNetAddr=ryan.jack@gmail.com
[1]
To=steven.tyler@vert.synchro.net
ToExt=1080
ToNetType=5
ToNetAddr=someone@gmail.com
[2]
To=ricky.bobby@vert.synchro.net
ToExt=190
[3]
To=austin.powers@vert.synchro.net
ToExt=418
ToNetType=5
ToNetAddr=another@yahoo.com

• The To value contains the recipients “name”
• The ToNetAddr value contains the recipients full email address
• The ToNetType value corresponds with SMB network types (0=None, 2=Fido, 4=QWKnet, 5=Internet)
• The ToExt value contains the recipient's user number on the BBS, when relevant

The mail processors are configured in the ctrl/mailproc.ini file.

Each mail processor is specified in a separate “section”, comprised of the mail processor name (or command-line) enclosed in square brackets (“[]”), followed by a list of optional “key = value” pairs.

Supported ctrl/mailproc.ini keys (for each mail processor) with default values:

If no Eval or Command key value is specified, the mail processor name will be used as the command-line to execute.

Non-JavaScript mail processors (i.e. native executables), must have the native key set to true.

For a mail processor to only process mail received for specific name(s), it must have a To key with a comma-separated list of destination names.

Example: To = listserver, listserv would cause a mail processor to only process mail received for either listserver or listserv.

If no To value is specified, then the mail processor will process all incoming (SMTP) e-mail messages.

The From key is similar to the To key: a list of zero or more strings to be used to determine which sender addresses the mail processor should be executed for.

The strings in the to/from lists may use the Synchronet .can style comparison patterns.

If a To value is specified, but is not a valid user name, then the PassThru key must be set to false or the mail may be rejected or forwarded (depending on the system configuration).

The filenames referenced below may all be modified or created by the mail processor.

Additional (read-only) variables available to JavaScript mail processors:

* See exec/mailproc_util.js and exec/listserver.js for example uses.

JavaScript mail processors may be implemented as either a single “Eval” string or an external JavaScript file (e.g. *.js).

External JavaScript mail processors (.js files) are loaded from the mods or exec directory if no path is specified on the command-line. If no file extension is specified on the command-line, .js is assumed.

Command-line specifiers (variables) are available for use in external mail processor command-lines:

The Synchronet Mail Server's SendMail Thread is responsible for delivering Internet email using the SMTP or ESMTP protocols, optionally authenticating using PLAIN, LOGIN, or CRAM-MD5 SMTP-AUTH methods if required by an SMTP relay server.

Both Direct Delivery and Relaying (e.g. through intermediary mail server) is supported.

The SendMail Thread currently can service one outgoing SMTP/ESMTP session at a time.

The SendMail Thread will attempt to deliver securely (encrypted via TLS) when possible, though it will fall back to plain text delivery when necessary.

The mail server also services authenticated “post office” requests from mail clients using the POP3 protocol, by default, on TCP port 110 and POP3S (Secure/encrypted POP3 over TLS) on TCP Port 995.

The POP3 server supports client authentication via the USER or APOP methods.

The POP3 server can service multiple simultaneous incoming sessions.

For IMAP support, see Deuce's IMAP Service.

The Synchronet POP3 server will filter-out SPAM-tagged mail automatically when a POP3 client logs in with <username>#nospam.

Append “#nospam” to your login name/alias and any SPAM-tagged messages will not be listed/downloaded. This is useful if for example you use POP3 to download mail to your phone and have limited storage or sorting options. The SPAM-tagged messages will remain in your inbox on the BBS so you should use another POP3 or local mail client to download and delete those messages (which could potentially include false positives).

The Synchronet Mail Server can be configured via SCFG->Servers->Mail Server:

╔══════════════════════════════════════════════════════╗
║                      Mail Server                     ║
╠══════════════════════════════════════════════════════╣
║ │Enabled                       Yes                   ║
║ │Log Level                     Informational         ║
║ │SMTP Interfaces               0.0.0.0, ::           ║
║ │SMTP Port                     25                    ║
║ │Submission Support            Yes                   ║
║ │Submission Port               587                   ║
║ │Submission/TLS Support        Yes                   ║
║ │Submission/TLS Port           465                   ║
║ │POP3 Support                  Yes                   ║
║ │POP3 Interfaces               0.0.0.0               ║
║ │POP3 Port                     110                   ║
║ │POP3/TLS Support              Yes                   ║
║ │POP3/TLS Port                 995                   ║
║ │Max Clients                   100                   ║
║ │Max Inactivity                30 seconds            ║
║ │Max Concurrent Connections    1                     ║
║ │Max Recipients Per Message    100                   ║
║ │Max Messages Waiting          200                   ║
║ │Max Receive Message Size      100M bytes            ║
║ │Default Recipient                                   ║
║ │Receive By User Number        No                    ║
║ │Receive By Sysop Aliases      Yes                   ║
║ │Notify Local Recipients       Yes                   ║
║ │Notify Offline Recipients     No                    ║
║ │Allow Users to Relay Mail     Yes                   ║
║ │Lookup Client Hostname        Yes                   ║
║ │Check Headers against DNSBL   Yes                   ║
║ │DNS-Blacklisted Servers       Throttle and Tag Mail ║
║ │Hash DNS-Blacklisted Msgs     Yes                   ║
║ │Kill SPAM When Read           No                    ║
║ │SendMail Thread...                                  ║
╚══════════════════════════════════════════════════════╝

... via SBBSCTRL->Mail->Configure:

... or by editing the ctrl/sbbs.ini file.

The [mail] section of the ctrl/sbbs.ini file supports the following configuration settings (key = value lines):

The Options key of the [mail] section of the ctrl/sbbs.ini file supports the following option flags (separated by a | character):

• TCP/IP FAQ
• sbbs.ini
• domains.cfg
• mailproc.ini
• dns_blacklist.cfg
• dnsbl_exempt.cfg
• spambait.cfg
• spamblock.cfg
• spamblock_exempt.cfg
• twitlist.cfg
• relay.cfg
• Example External Mail Processor (JavaScript)
• ListServer
• ListGate
• spamc
• Servers