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Brian Cryer's Glossary of IT Terms with Links

Backup Mail Server

Backup Mail Server
A secondary email server to which emails are delivered should the primary email server be offline. A backup email server will then forward emails to the primary email server once the primary email server comes back on-line.

When an email server is down most sending email servers will retain the email and try sending again later. Generally most servers will give up and issue a non-delivery notification (NDR) to the sender after about two days (the actual time varies between servers). So even without a backup mail server emails will normally be delivered if the mail server is down for a short period of time. A backup mail server provides resilience where:

  • The sending server has a very short timeout before giving up and issuing an NDR.
  • The server will be down for a long period - as a guide anything over a day or so.

In these scenarios a backup mail server will receive emails and ensure that they are delivered once the primary mail server comes back on-line.

cf SMTP, MX Record.

For more information see: