POP3 server error 5

I have just received a mail from a user reporting that, in mail-app of mine, all at once he gets the following message error (generated in serverError):
“5 -ERR (SYS/TEMP) problem retrieving message”.
I’m not sure if he uses POP3 simple or secure. I’ll have to ask him.
On the other end, has got other accounts, all working fine. Only this particular one generates the error when trying to get mails’headers.
BTW in the last several years, I (i.e. the app) never encountered this particular error.
Any idea where I should look?

DuckDuck fast revealed the following Case in the Mozilla Support system. Maybe it will help?

It seems exactly the same. Meantime I’ll tell the fellow to clean the spam folder, than we’ll see if things go back to normal.
Thank you, Shasha.