A few hrs after upgrading GWIA started bouncing all external email

Message #XXXXX to user@domain bounced by destination server. Reason: 5.1.0 - Unknown address error ('530', ['Authentication required']

A restart of GWIA resolved that for a few days then it started again but this time restarting GWIA did not resolve it.
We were not using the –forceinboundauth switch in the gwia.cfg so this did not make any sense to us. So we came up with a workaround solution. Use the –forceinboundauth switch.
We still do not have any idea why. I hope it works and is not just a coincidence. Just putting this out there in case someone has the same issue or know why this happened.