E mail

With a [mail] section each user has an e-mail address, and their BBS mail is their e-mail. A letter written on a 3270, a modem or the web goes out to the internet, and mail from the internet arrives in the same inbox the CompuServe and BBS mail commands read. Mail programs (Thunderbird, Apple Mail, mutt, K-9) read it over IMAP or POP3 and send over SMTP, logged in with the BBS name and password.

; bbs.conf
[mail]
domain = example.org
hostname = mail.example.org
dkim_key_file = /etc/bbs/dkim.pem
cert_file = /etc/bbs/cert.pem
key_file = /etc/bbs/key.pem

[imap]
address = :993
tls = implicit

[pop3]
address = :995
tls = implicit

[smtp]                       ; submission: users send mail, logged in
address = :587
tls = required

[mx]                         ; mail from the internet
address = :25

Every server runs only if its section is there. All of them use the BBS's guard (bans, throttling), and each has max_clients, max_per_ip and idle_timeout. tls is no, optional (STARTTLS offered), required (no login before STARTTLS) or implicit (TLS from the first byte). Ports below 1024 are bound before the BBS drops root, as the others are.

Addresses¶

A user's address is their name in lower case, with spaces as dots, at the domain: Alice Smith is alice.smith@example.org. bbs mail address NAME prints it. +tags are ignored (alice.smith+lists@ reaches Alice). postmaster@, abuse@, hostmaster@ and webmaster@ go to [mail] postmaster, or the first sysop.

When writing mail on the BBS, the recipient can be a user's name, a FidoNet address (Name@1:234/5) or an internet address (bob@example.net). In CompuServe Mail, >INTERNET:bob@example.net works as it did there. Mail to FidoNet's internet form (john.doe@f5.n234.z1.fidonet.org) goes as netmail.

Folders¶

Mail is kept once. Each user has their own entry for it, with its read and starred marks, so the same mail sent to five users is stored once. Every mail program sees the same folders:

Starred, Unread and All Mail are views: starring a mail in the inbox puts it in Starred, and taking it out of Starred unstars it. Deleting moves mail to the Trash, where a mail program can restore it (by moving it out) or expunge it. Mail in the trash longer than trash_days (30) is removed for good. Folders cannot be created or renamed.

POP3 sees the inbox. Mail retrieved is marked read, and DELE puts it in the trash.

Sending to the internet¶

[mail] outbound says how mail for other domains leaves:

• direct (the default): the BBS looks up each domain's MX and delivers on port 25, with STARTTLS when the other side offers it. Many home and cloud networks block port 25 out, and receivers want SPF, DKIM and a reverse DNS name for the host, so this needs some DNS work (below).
• smarthost: everything goes through another mail server (your provider's, a relay such as Postfix, or a service like Mailgun), given in [smarthost]. That server does the delivering. This is the easy route when one is at hand.
• none: only mail between users here, and FidoNet.

[mail]
domain = example.org
outbound = smarthost

[smarthost]
address = smtp.example.net:587
tls = starttls               ; or implicit (465), or no
username = bbs@example.org
password = secret

Mail out waits in a queue, tried again with longer gaps (five minutes, then up to four hours) for queue_days (5). When a server refuses a recipient for good, or the days run out, the sender gets a note in their inbox saying why. bbs mail queue lists what is waiting, and bbs mail send tries it all now.

DNS for a domain of its own¶

For mail to arrive, and to be believed when sent directly:

example.org.              MX  10 mail.example.org.
mail.example.org.         A   203.0.113.5
example.org.              TXT "v=spf1 mx -all"
bbs._domainkey.example.org. TXT "v=DKIM1; k=rsa; p=..."
_dmarc.example.org.       TXT "v=DMARC1; p=quarantine"

and a reverse DNS (PTR) name for 203.0.113.5 of mail.example.org, which the hosting provider sets. bbs mail dkim /etc/bbs/dkim.pem makes the DKIM key and prints its record. With a smarthost, use the SPF and DKIM its provider gives instead.

Mail from the internet¶

[mx] takes mail for the domain on port 25, without a login, and only for users here: it is never an open relay. With checks = yes (the default) it checks SPF, DKIM and DMARC, and adds an Authentication-Results header. A mail is refused when it fails a DMARC policy of p=reject, or, for a domain with no DMARC record, when SPF fails and no signature of that domain holds.

Spam: DNS blocklists¶

A DNSBL is a list of addresses known to send spam, asked by DNS. [mx] dnsbl names the lists to ask; none are asked unless it is set:

[mx]
address = :25
dnsbl = zen.spamhaus.org, bl.spamcop.net

The connecting address is looked up in every list at once, at the first MAIL FROM. If any list has it, the mail is refused with 554 5.7.1 and the list's reason (its TXT record, often a link where the sender can ask to be taken off), and the refusal is logged. A list that does not answer in ten seconds, or answers with an error, refuses no one: mail is not lost because a list is down. Addresses on this machine and private networks are never looked up, and nor are users sending through [smtp], who log in: a user on a listed home connection can still send.

Spamhaus answers nothing useful to queries through public resolvers (8.8.8.8, 1.1.1.1, a cloud provider's): the BBS logs refused the query when that happens. Run a resolver of your own (unbound, or the system's with no forwarders) on the mail host, and see Spamhaus's terms: heavy or commercial use needs their paid feed. Lists to consider: zen.spamhaus.org (Spamhaus SBL, XBL and PBL in one), bl.spamcop.net, b.barracudacentral.org (free, after registering the resolver's address with them).

DNSBLs only look at the sending address. SPF, DKIM and DMARC (above) check that mail claiming a domain comes from it. Together they keep out most spam; for content filtering (SpamAssassin, rspamd) put Postfix in front and hand mail in by LMTP, below.

LMTP¶

If another mail server already takes the domain's mail (Postfix, with its own spam filtering), it can hand mail to the BBS by LMTP instead:

[lmtp]
address = /run/bbs/lmtp.sock

# Postfix main.cf
virtual_mailbox_domains = example.org
virtual_transport = lmtp:unix:/run/bbs/lmtp.sock

On the terminal¶

Mail from the internet is shown as text: HTML mail is turned into plain text, and attachments are listed by name and size (they stay whole for mail programs). Messages larger than max_message (10M) are refused.

Databases¶

Mail lives in the BBS's database, whichever it is: SQLite, PostgreSQL, MySQL or MariaDB. bbs db copy carries the mail, the folders and the queue along with the rest.