I just ran into this same problem after my board was up for over 2 years. Was using the local mail service and it stopped working today with the same debug error on line 234.
Isn't mail local to the forum, when it is selected to not use smtp. Why would it break all of a sudden?
I turned on SMTP and it is working fine again. Did a table fill up or are there limits to number of emails that could be saved?
What is recommended? We have a pretty active board with over 1800 visits a day.