Hack the WinterMute: 1 (CTF Challenge) - Hacking Articles
SMTP Server Status Codes - Answers That Work
SMTP Protocol Error: 501 5.1.7 Bad sender address syntax when sending mail from our printer through Nethserver - Support - NethServer Community
With email in outbox: Failed to send some messages. Negative SMTP reply 501 5.1.3 Bad recipient address syntax · Issue #1328 · thunderbird/thunderbird-android · GitHub