Oh, sorry for being so concise 
My set up is so simply:
Synology NAS with Librenms docker.
The Synology firewall is off.
Moikrotik router.
All output traffic it’s allowed.
Maybe I can try with the Google SMTP server 
When I click the test I obtain this 
But, if I configure a printer to alert directly, with the same config, the printer alert correctly.