The user is deleted and not delimited. So he won't be in user master table USR01. They delete the users I think to comply with ISO certifications; It's a requirement from our security team. I probably should check with them why they delete them (not sure if they can actually delimit users though. I know that they can "lock" the user which is like freezing him but not sure if that also means delimiting.
The reason you probably never encounter these errors is because the mail step won't fail if you send it a delimited user or not too many users are being deleted while the workflow is in process.
The error would be that sendtaskdescription cannot be executed. When I manually replace the user in addressstrings (which was deleted) with another one and re-start the workflow then it works.
Thanks,
Ali