Retype the SMTP email and password in the sys admin module.
Then go to their user options and retype their email address.
This resolved our issue with this same problem.
Hotfix KB 4526351 may help but we ended up fixing it via manual workaround due to the time sensitivity. The following solution worked for both EFT payment advices via print management as well as user workflow notifications:
1. Remove SMTP settings in system admin and re-entered it along with the p/w (System administration > Setup > Email >Email parameters > SMTP settings)
2. Re-select SMTP and updated e-mail address with all lower case letters (sysadmin > User > User options > Account > Email provider section)
Also note that you can check email sending status here... where you can see when they stopped and started up again to validate once it has been fixed: System administration > Periodic tasks > Email processing > Email standing status
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