Can’t send an attachment with Outlook Express

I have a PC w XP SP3 using Outlook Express. When using the PC at home I am not able to send email with an attachment. If I remove the attachment, email goes through fine. With attachment I get a SMTP server timeout error.

I brought the PC into my office, same ISP as home, different router, email with attachment goes through without a problem. Im stumped. Can anyone help?