How to read an SMTP code
The three-digit code: 2xx success, 3xx the server waits for more, 4xx a temporary error (it will be retried), 5xx a permanent one (the message comes back). An enhanced code like 5.7.1 adds the reason: the middle digit is the subject (address, mailbox, network, policy), the last one the detail. The text after the code differs between servers, but often holds the key: a blocklist name or a help link.
Check a domain’s email setup end to end: domain check, deliverability test, header analyzer.