Phone problem

[#1]

I have had two email accounts set up since joining Plusnet about five years ago.
The main one is "username" and the second is "username+other".
Two years ago, I got my first smartphone, and I started using that to deal with email while I was out and about.
This week, it suddenly started picking up only the "username+other" mail.
Everything still works fine on my desktop at home.
I have checked the settings (pointless exercise as nothing has been changed) and I have switched the phone on and off, then deleted the "username" account from it and re-entered the details.
But it still functions only with the "username+other" account.
Can anyone make any suggestions?

[#2]

Which phone are you using? I'd start completely from fresh and remove all email accounts from the phone then set up just the "username" account and see if that works first and take things from there.
Adam

[#3]

It's an HTC Wildfire S.
In fact, I've done this twice, starting from scratch, and each time it cures the problem for a while, but it then recurs.
Each time, it's just one of the two email accounts.