There could be a combination of 2 reasons for this.
- The server is designed to block attachment types which are seen as a danger if they are not zipped up. Some blocked attachment types that are: .bat .src .exe .pif. 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. Unfortunately, there is no way to turn this off so your only option is to zip your attachment.
- Another reason could be the size of the attachment. If you send an attachment 10MB and larger, it will fail due to the size.
If you send it from webmail this limit is less than 2 MB. A better option to get round these limits would be to upload the file to your site and provide that link to the file instead. You could even password protect so the public cannot view or access the file.