1. The server is designed to block most attachment types if they are not zipped up. 
A few of them being blocked are... .pdf .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 unfortunately there is no way to turn this off so your only option is to zip your attachment or doing the below.

2. 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. 

If you are sending it from webmail this limit is less than 2mb. 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
Was this answer helpful? 278 Users Found This Useful (566 Votes)