You should not experience issues while sending attachments with encrypted mails. However, some e-mail applications sometimes handle encryption in a slightly non-standard way and may experience issues when receiving files from other clients.
Of course, it is difficult to give you a very precise answer without having a look at the installations on both ends... May I suggest that you make sure that both you and your friends use the latest versions of your e-mail clients ? Also, they may want to try alternative e-mail applications like Mozilla Thunderbird -- even temporarily, as a test, to isolate the issue.
Would you still be unable to send encrypted attachments, may I suggest that you post on the Apple Discussions ? That way, many experienced Mail users will be able to help you...
Let me know if this helps !