Notifications stop working for no reason

An issue where previously working notifications just stop working.

No they are not in the Spam folder.

Also the email accounts function perfectly well receiving alerts from other systems.

After what seems to be a random number of days the alerts start working.

If I define a new user with a new email address it works immediately receiving alerts. Obviously an unacceptable workaround.

Seems to be an issue with the Vera Cloud backend.

Any ideas as to a fix?