I think the authors of SMTP had a healthy attitude towards server uptimes:

   Retries continue until the message is transmitted or the sender gives
   up; the give-up time generally needs to be at least 4-5 days. 

I saw an application once about 25 years ago that was built on SMTP for communication. I think it was an inter library loan system, but can't remember for sure. I was a pretty smart way of not having to worry much about redundancy in the communications.

At the time, most email was either local to the host (big mainframe in the basement) or transferred once a day through scheduled dial-up connections during off-peak phone hours