421 Service not available, closing transmission channel
When an SMTP mail server responds to a mail message with:
421 Service not available, closing transmission channel
This can be a reply to any command if the SMTP server knows it must shut down. It typically occurs when the mail server is shutting down (e.g. due to scheduled maintenance)
When this occurs, mail sending has failed and will need to be retried at a later time.
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Also in this topic:
- 250 OK Response
- 552 Requested mail action aborted: exceeded storage allocation
- 550 Mailbox not found
- 454 4.7.0 Temporary authentication failure
- 530 5.7.0 Authentication required
- 535 Authentication credentials invalid
- 452 Too many recipients
- 452 Requested action not taken: insufficient system storage
- 451 Requested action aborted: local error in processing
- 450 Requested mail action not taken: mailbox unavailable
- 251 User not local; will forward
- 554 5.3.4 Message too big for system
