You may be sending mail to a Public Folder on your domain, and that’s pretty cool. However, if you try to send a sizable attachment with that message you will run into trouble.
The usual setting for Anonymous under Properties > Permissions > Client Permissions is Contributor. This gives Anonymous (e-mail) the ability to create items in the folder. If you attach a tiny GIF or JPEG it’s no problem. The message is accepted and it sits happily in the Public Folder. But if you send a large JPEG or PDF (multiple MB worth) you will get an SMTP time-out or perhaps some other equally meaningless error.
To remedy this you must give Anonymous (or the e-mail address sending the file) Read permissions as well.
Why read? Perhaps “receive an attachment from” is like “I allow you to read the attachment you have sent to me as I store it for you” or some bullshit. I don’t know. Just do it, ok.