What is your account type - POP or IMAP?

thanks for answering - pop- but I fixed it-- apparently the mail.dat file was corrupted and when I deleted it it worked properly again- the question is- how does this get corrupted? Found this on the help site. If this is a known issue, why can't it be fixed permanently (rhetorical question)

Thunderbird doesn't have a mail.dat file. Do you mean popstate.dat?

yes-that's right- answered from memory which isn't always accurate!