Mail: problem with attachments

Hello.
In the past few days I'm experiencing some problems with Mail: I can send emails with no attachments, but when I try to attach a photo or a document, the sending process starts, then slows down immediately, and stops just before completion. I get stuck in between and it doesn't resume anymore.
Why?
Thank you!

Who is your mail provider and ISP? Some restrict attachments of a certain type or size. That's not an Apple mail problem.

Regards.