Don't use maintenance plans would be one of the best ways in my opinion. They just seem to have so many issues that I can't see how they save time for anyone. Did you check the send mail logs? You should be able to find the mail  that was sent with that:
select *
from msdb.dbo.sysmail_allitems

You should be able to take a line out of the emails you get to search on the email as well. The body column of sysmail_allitems has the text of the email that is sent. I'd check that to make sure it's coming from the server you think it is coming from.
You could also query sysjobs where notify_level_email > 1 to check the jobs that are sending out notifications.
You should have some clues by querying those. It's likely the job is still there or the alert is coming from another server, environment.
Alerts aren't sent out from jobs that have been deleted so you may want to proceed as if the job does exist somewhere and find where the alert is coming from.

Sue