The Engineering Challenges:
Finite State Machine (FSM): SMTP is strictly stateful. I implemented an FSM to enforce command sequencing (e.g., preventing DATA before MAIL FROM). It ensures that protocol violations are caught at the socket level with proper 503 Bad Sequence codes.
Buffer-Oriented Processing: Used bufio.Scanner to handle the byte stream. The biggest hurdle was the DATA phase logic—properly detecting the \r\n.\r\n sequence while managing memory efficiently using strings.Builder.
Concurrency: Leveraged Go's Accept() loop to spawn independent goroutines for each session, ensuring that the relay latency to Gmail (via STARTTLS) doesn't block the listener.
ISP Workarounds: Configured to run on port 2525 by default to bypass the common ISP block on port 25.
Status Codes Implemented: I implemented a subset of RFC 5321 codes, including 220 (Service Ready), 354 (Start Input), and error handling for 501 (Syntax) and 451 (Local Error).
Why I built this: Most modern tutorials stop at "How to send an email with a library." I wanted to see how the "dot-stuffing" mechanism worked and how a server actually negotiates a multi-step handshake over a raw connection.
I’d love to hear about edge cases I might have missed—specifically around handling malformed headers or managing long-lived TCP connections under load.
Source Code: https://github.com/Jyotishmoy12/SMTP_Server