troubles sending attachments

[Potvaliant]

We use a T1 to connect two offices. Our head office contains the POP3 email server. The users in the other office are having a lot of trouble sending emails with attachments. There is a mixture of Outlook and Outlook Express users, the messages seem to sit in the outbox and only sometimes it will be delivered, but it is rare.
I think that the T1 is overloaded with traffic and is causing the problem.
If there is anyone that has encountered a similar problem or may have alternate explanations I would appriciate any help.
Thanks