通常、Rails の production ログにジョブの例外エラーが記録されます。例えば以下のようなものです。
linux# docker exec -it your-app bash
your-app:/# cd /shared/log/rails
your-app:/shared/log/rails# grep -i SMTP *
production.log-20201215:Job exception: 454 4.7.0 Cannot authenticate due to temporary system problem. Try again later. m7sm3825233oou.11 - gsmtp
production.log-20201215:Job exception: 454 4.7.0 Cannot authenticate due to temporary system problem. Try again later. 63sm3561399otx.2 - gsmtp
production.log-20201215:Job exception: 454 4.7.0 Cannot authenticate due to temporary system problem. Try again later. z63sm4258403otb.20 - gsmtp
production.log-20201215:Job exception: 454 4.7.0 Cannot authenticate due to temporary system problem. Try again later. q18sm3815880ood.35 - gsmtp
production.log-20201215:Job exception: 454 4.7.0 Cannot authenticate due to temporary system problem. Try again later. h2sm4265076otn.15 - gsmtp
production.log-20201215:Job exception: 454 4.7.0 Cannot authenticate due to temporary system problem. Try again later. y84sm4346372oig.36 - gsmtp
production.log-20201215:Job exception: 454 4.7.0 Cannot authenticate due to temporary system problem. Try again later. 8sm4343514oii.45 - gsmtp
production.log-20201215:Job exception: 454 4.7.0 Cannot authenticate due to temporary system problem. Try again later. t18sm722516otc.64 - gsmtp
production.log-20201215:Job exception: 454 4.7.0 Cannot authenticate due to temporary system problem. Try again later. j22sm1840062otp.45 - gsmtp
したがって、上記のように Rails の production ログから手がかりが見つかる可能性があります。