This company have two distant office buildings in a big industrial yard, accessing internet with two different internet connections.
In the morning the guy in site office, which is managing the yard at the back, called to report an email setup issue when he’s trying to update his email password and setup that email account on his new work phone.
He came to the main office with both his phones. We changed his password with admin console and setup his phones with new credentials. Both worked. And then we went to site office to deal with the email on his laptop. Updating with new password didn’t work. Then I noticed the email just setup on his phones also stopped working. With the idea in my mind I used my device to connect to the site office wifi and sent a test email. It failed to send out.
The last test was on his newly setup phones. After turning off wifi, the email worked on that phone, using mobile data. Now it’s clear the issue is from the wifi. With little efforts we restarted the router providing wifi in site office. The issue is gone.
Apparently the guy changed his password at a wrong time when the entire site office lost the connection with email server somehow. Anyway, had some good luck on this one.