Sent email missing attachments

Since I upgraded to OS 10.4.11 I have been having a problem with my emails to others missing the attachments. They swear my attachment is not there although it is marked 'sent' on my side and I get no error or bounce back from any servers. When I go back to make sure I attached the item it is there in my mail. These are small image attachments that should not hit size limits (630k-5mb). This is embarrassing! It seems to be intermittent (mailing to the same person, sometimes they get it sometimes they don't) and only happens when I send to people on PCs from my MAC. Any advice? Is it my problem or their problem?