First, you should let us know you have a problem because if you can't download your mail, incoming mail will continue to accumulate until your box becomes full. At that point, no incoming mail will be accepted. Secondly, if we have to reset your mailbox, you will lose all the mail in it. The longer you wait, the more mail will accumulate and, hence, possibly be lost. The first thing to try is to go through your settings and make sure you have settings which will allow all mail to download.
Most mail clients have restrictions on the size of messages, etc.
If that doesn't work, try another mail client. You can download Eudora Light for free and it is one of the best free mail clients. Pick a mail client other than the one you are having trouble with, install it and see if that helps. As a last resort, we can clean out your mailbox for you. This is a manual process and we cannot differentiate between messages in the mailbox.