Mac Mail--Emails with Sender Unknown/ No Subject

On one of Mac Mail email accounts stopped displaying my emails.

The mail activity displayed showed that emails were being received.

When I ran account info on this mail box- there were several

emails labeled with Sender Unknown and NO SUBJECT. These

were the emails that were received but not displayed.

I know that these are valid emails since they appeared on my ipad.

Any idea what the problem is? My other mail accounts have

no problems--furthermore when I deleted the Sender Unknown emails

from the POP server using the remove option on account info-

it appeared to correct the problems for future emails.