- What port does SMTP operate on?
SMTP does mail transmission over TCP port 25. Many providers use alternative ports for mail submission.
- How do you determine which SMTP server receives mail for a given domain?
The mail servers responsible for receiving mail for a given domain are given by MX-type DNS records.
- How do you determine which SMTP server sends mail for a given provider?
It's not possible to determine that in the general case. In any case, several servers could be responsible. Sometimes these servers will be listed under a TXT-type DNS record using SPF, but that is certainly not universal.
- Why won't an SMTP server relay mail without authentication?
Open relay SMTP servers are targeted by spammers. SMTP servers require authentication to prevent abuse.
- How are binary files sent as email attachments when SMTP is a text-based protocol?
Binary files must be re-encoded as plain text. The most common method is with Content-Transfer-Encoding: base64.
