I have two smtp connectors for outbound mail in the same org, the connectors are in different admin groups with cost 1 and cost2 for the entire org.
I wanted the second connector to be used if connector 1 was unavailable. During a recent maintenance window I was able to test this config and it was not successful from 3 sites.
The outbound mail was stuck in the queue on 3 sites, on 2 sites mail did flow out the second smtp connector.
Site 1 1 backend server
Site 2 1 backend server - 1 frontend server smtp connector cost 2
Site 3 1 backend server
Site 4 2 backend servers - 1 frontend server smtp connector cost 1
I took smtp connector cost 1 offline. On Site 1 and Site 2 mail did route through successfully through the smtp connector on site 2. On site 3 and site 4 mail queued and did not route mail queue on both until I brought the smtp connector cost 1 online.
Any input on this would be great why this didn't work on site 3 and site 4.