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If you get an email back and it says "undelivered" or "undeliverable" or something like that, does that always mean that the email address you sent it to is no good, or could there have been a different issue?
Technically, it means that your outgoing mail server was unable to send the email. There're a few different reasons why an email would be undeliverable:
- the email address is invalid
- your outgoing mail server is unable to contact the destination email server
- the destination email server is unable to route the email to the email address