Editing Training 2022 Linux team/ (section)

==Training on Mail Sending & Receving Problem & Bouncing Problem Part-6 . Dated 08 Feb 2022 - Tetra Support Staff - Biswajit Banerjee==

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'''Video summary:''' "Training Sending,Recieveing Bouncing part6" -- continues the bounce-diagnosis series with more live examples (interrupted partway by an unrelated live infrastructure task the presenter was simultaneously monitoring -- a data-sync operation being tracked in parallel). Covers reverse-DNS (PTR) records as another critical factor in deliverability -- checking that a relay server's IP resolves back to the expected hostname, since receiving mail servers commonly reject mail from IPs with missing/mismatched reverse DNS. Walks through an SMTP 554/571 "sender address rejected" bounce for a SysNet Global user, and makes an important diagnostic point: seeing "sender address rejected" from Tetra's own relay server (not the destination server) doesn't just mean that one recipient domain is blocked -- if the sending mail server is configured to route ALL outbound mail through a dedicated relay, then a sender-address rejection at the relay level blocks that user's mail to everywhere, not just the one destination they were testing with, which matters for correctly scoping the ticket. Explains the two-file Postfix relay configuration structure used across Tetra's relay servers: a "network" file whitelisting which IPs are allowed to relay mail through the server, and a sender-restriction file where specific sender addresses can be explicitly listed with `reject` to block them from using the relay (found to be the actual cause of the case being debugged -- the user's address had been deliberately blocked at some earlier point). Also covers reading Postfix's log symbols for identifying bounce/mailer-daemon messages (a message with no outbound queue ID reaching a "mail delivery subsystem"-style sender is characteristically a bounce), an oversized-attachment bounce example, and closes by answering a trainee's question distinguishing CBPolicyd (the milter/policy daemon Tetra primarily uses for quota-style sending limits) from milters more generally, noting CBPolicyd or other milters can also be configured to reject based on content, not just volume, depending on setup.