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. An example of this is the .eml extension; In this case try sending your attachment in a different format.
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