I was looking into creating a backup mx server for a high availability mailcow setup. It seems that this is not easily done. While researching to find out how long an average SMTP server keeps trying to send to a server that is down I found that RFC 5321 advises at least 4 to 5 days. Mailcow has a custom setup of 1 day, which is very short. The user will be unaware for 5 days that his mail has not been delivered, which can be negative. But I still would like to follow the advice of the RFC.
RFC 5321, in section 4.5.4.1, has this to say:
Retries continue until the message is transmitted or the sender up; the give-up time generally needs to be at least 4-5 days. It MAY be appropriate to set a shorter maximum number of retries for non-delivery notifications and equivalent error messages than for standard messages.
Postfix default is also 5 days: http://www.postfix.org/postconf.5.htmlhttps://tools.ietf.org/html/rfc5321#section-4.5.4
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