The server is designed to block attachment types which are seen as a danger if they are not zipped up.
A few of them being blocked are: .eml .bat .src .exe.
The reason for this, is that your mailbox would be flooded every day with viruses if this was not in place. A few attachments that are fine without zipping up are .gif .jpg .html .pdf .png.
Unfortunately, there is no way to turn this off so your only option is to zip your attachment or if you are sending it from webmail, this limit is less than 10mb. A better option to get around these limits would be to upload the file to your site and give out the link instead. You could even password protect so the public cannot view or access.
You can also upload your files to Google Drive, OneDrive, Dropbox, Box, among other cloud sharing services, and share the link with others (and even password protect the link). Cloud sharing services allow for much larger file sizes.
Another reason could be the size of the attachment. If you're sending a 10mb or larger attachment, it will fail due to the size.
Attachment sizes are changing all the time so ask us to verify for you.