It sounds like this is an issue with your email service rather than with Firefox. We might not be able to help you out.

Firefox doesn't do email, it's strictly a web browser.

If you are using Firefox to access your mail, you are using "web-mail". You need to seek support from your service provider or a forum for that service.

If your problem is with Thunderbird, let us know and we can move this thread to the Thunderbird queue. This question currently is in the Firefox queue for answers.