Trouble submitting PDF form with Web Mail

Hello,

I have a fillable PDF form that I'm trying to submit. When I submit the form through an email client (Outlook, Mail, etc.) it works perfectly. When I try to submit using the Webmail option, I get this message: "An unknown error has occurred while uploading the attachment. If you are not sure how to proceed further, you can save your form and return it manually using your Internet mail service."

I've checked the file thoroughly and everything looks correct. I'm perplexed as to why it submits just fine through an email client, but won't submit via Webmail. I've tried on a Mac and a PC and I've tried using a Yahoo account and a Gmail account with the same results. Any ideas?

Thanks!

Kate