Recipients not receiving my attachments via email

A problem has recently come up with my email recipients not receiving emails that have attachments. The attachments are rather small:

33KB or 139KB. I never get a message saying the email wasn't received. I sent something on a Monday and on a Friday my contact said he never received it. This has happened around five or six times with different people. In the past, I have sent attachments up to 3MB without problems. What could be wrong and can it be corrected?