Email attachment restriction

https://support.google.com/mail/answer/6584?co=GENIE.Platform%3DDesktop

Attachment size limit
You can send up to 25 MB in attachments. If you have more than one attachment, they can't add up to more than 25 MB.
If your file is greater than 25 MB, Gmail automatically adds a Google Drive link in the email instead of including it as an attachment. 
The person you send the file to can download it so they'll have access to it even if you remove it from Google Drive.

To prevent viruses, Gmail doesn't let you attach executable files, like files ending in .exe. 
If you try to attach an executable file, you'll see the message "Blocked for security reasons."

https://www.fastmail.com/help/account/limits.html

Attachments with double file extensions (e.g. filename.doc.exe) are not allowed, as they are commonly used for virus transmission.

The same maximum size limit of 70 MB (due to encoding issues this usually means an attachment of 50 MB) applies to received email as well.