BL4's SMTP server

BL4's SMTP server is an inbound only server that uses hardcoded values to handle email. The server stores incoming emails in various text files. For those who may not know, Simple Mail Transfer Protocol (SMTP) is the standard protocol for email transmission over the internet, and SMTP is relatively simple as it is a text-based protocol that includes recipients' information and the message to be transferred. Testing an SMTP server is easy with the telnet program, and SMTP uses TCP port 25.

To find the SMTP server for a specific domain, Mail eXchange (MX) DNS records should be used. Initially developed as a complement to UUCP in the early 1980s, SMTP became widely used when both sending and receiving machines of an email were consistently connected to the network. Sendmail was one of the first mail transfer agents to implement SMTP, and it was followed by at least 50 other programs, such as exim, Postfix, qmail, and Microsoft Exchange Server.

Moreover, since SMTP was entirely text-based, it was not effective with binary files. To tackle this, Standards like MIME were developed to encode binary files for SMTP transfer. These days, most SMTP servers support the 8BITMIME extension, making transferring binary files almost as easy as plain text. SMTP is a "push" protocol, which means it does not allow the user to "pull" messages from a remote server. To do this, a mail client must use POP3 or IMAP, while another SMTP server could use ETRN to trigger a delivery in SMTP.

Lastly, BL4's SMTP server timeout feature was added in the latest update, which removes idle connections after a preset length of time, making it a must-have SMTP server.