Thought I would point out a minor issue that we've been experiencing every time we upgrade SmarterMail (has been ongoing with every update since 16.X).
After exactly 21600 seconds of upgrading SmarterMail on our primary server our Incoming Gateways (set to "Domain Forward" with "Enable SmarterMail Gateway Mode" and "Web Service" User Verification enabled) all of a sudden stop delivering inbound email to SmarterMail.
A restart of the SmarterMail service on each of our three Incoming Gateways resolves the issue.
As this occurs with very specific timing I'm assuming that this is caused by a caching timeout of the User Verification. Not sure why it isn't able to do live User Verification lookups with the primary server post-upgrade until the service is restarted on the Gateways though. There definitely seems to be a disconnect somewhere.
Normally, remembering to restart the service on all Incoming Gateways is something that should be entirely on me (just like always remembering to reset the Recovery Options for mailservice in the Services.msc to automatically restart the service upon failures which always gets overridden upon every upgrade). However, as our Monthly Scheduled Maintenance Window is 2am-3am it's not unusual for my tired and overworked brain to forget especially since all initial tests show the upgrade as successful and everything is working perfectly as expected. Yet inevitably after every upgrade I am awoken at 8am-9am to numerous Zabbix alerts, panicked/overwhelmed Support Technicians, and angry customers wanting to know why they aren't getting inbound emails. So, I thought I would mention it in case SmarterTools wasn't aware of this fringe case and minor QoL issue.