thank you for your answer. As you assumed I use input rules but also other devices connected to RUT by LAN/WLAN which are able to send emails or have a build-in FTP client. For this, the RUT should offer a FTP server and (even better) a MTA like Postfix. As you mentioned, I use a mounted USB stick for persistence, to save internal RUT flash memory.
The home of the FTP users and the MTA storage could be located to USB storage and everything is fine.
If you add an MTA by default, all your build-in retry count & interval configurations at each event source (like input rules) will be obsolete if you configure localhost as SMTP server. The same for all devices in RUT network, they use RUT as SMTP server. It is always available :-)
Then, in the outbox of the MTA (the sendmail part) needs a retry config only, not every event source any more. Also the MTA outbox can be configured to forward incomming mails to specific destinations. This email routing option is also centralized now and don't need to be implemented at every mail source again and again...