National/special characters in the message text are corrupted.
This is most likely caused by a bug in the recipient's software. Apple Mail uses a variety of standards for encoding characters some of which are not supported by other e-mail software.
Workaround: If it seems that the problems are triggered by including attachments, try inserting all attachments at the end of the message without any spaces or text between and after them. This will produce a more compatible message. It is also more reliable to transfer larger and more complex text documents as attachments instead of putting them in the message body.