Here is a few items to check.
- Check to see in your POP3/SMTP Email Server Profile if your settings are the same as follows.
-Incoming server location = pop.gmail.com
-Outgoing server location = smtp.gmail.com
-Authenticate using = set to "Credentials Specified by a User or Queue"
-Incoming Port = 995
-Outgoing Port= 587
'This is the outgoing port that works in my instance if the problem persist afterwards to the outgoing then this would be the first place I would look'
Afterwards head to your specific users mailbox. Make sure that you specify your credentials and also set Incoming and Outgoing to Server-Side Sync and Appointments, Contacts, and Task to None (This is not available with POP3), afterwards click save and then finally Test & Enable mailbox. A dialogue box will pop up letting you know the test will run and a checkbox will ask "whether to sync items with exchange from this Dynamics org only?" Make sure this is checked.
Another small but problematic setting is on the two-step verification on the email clients side, a quick google search should show how to disable this setting.
Hope this helps