Can't reply to correct account of two

I have two different eMail accounts at, but when I try to reply, the FROM picks the other account and I can’t change it with the drop down menu (only one account appears).

Can you help?

I “think” I found the problem, both IMAP and SMTP have to be checked:

At least when I changed it, it seems to work.

The other account should have been grayed out so at the very least, one would know there is a problem. It should also have a mouse-over tip at the check boxes where IMAP/SMTP are selected.