AkeWallin Everything seems to be correct, because you can send mails without attachment.
Let’s have a look at some things:
A) Call the command “Tools, Options, Mail Format” in Outlook. Have you set HTML there? If not, please choose HTML.
B) Do you have an antivirus software installed in which is set that on Outlook it should scan the attachments? Can you deactivate your antivirus once for a test dispatch?
C) Do you have the error only with a certain e-mail address as or does it not matter which e-mail address you write to?