You might not be able to send mails with attachments because of:
- A local antivirus / security application running on your local computer. It might detect that the attachment is either not permitted or is infected by a virus. In this case, try disabling your application and resending the attachment;
- The file extension is found in the forbidden extensions list on the server. For example, the .eml extension; In this case, try sending your attachment in a different format.