MAIL TIME-OUT WHEN INCLUDES A BINARY ATT

Here’s our problem,

We are serving our own mail on at NT server using Eudora WorldMail. Have recently changed Internet providers. Prior to the change we had no trouble as spelled out below…

We can receive any kind of mail if we are on our own LAN or if we dial in to our own LAN. However, when anyone tries to receive their mail from the outside world, (via cable modem, ISDN, or DSL connection), they get timed out IF the mail message includes a binary attachment.

Does anybody have a clue as to the answer to this problem?

Any help will certainly be appreciated