Emails to a SharePoint 2007 document library have some "gotchas" when it comes to attachments. Filenames and encoding can both cause silent failures; the email appears to be sent correctly, but the doclib is unchanged, and no error appears to the user, either via return email or on the website.
Filenames may contain characters that are valid in your desktop OS, but are not valid for Sharepoint. Microsoft lists the bad guys in this Knowledge Base article:
Information about the characters that you cannot use in sites, folders, and files in SharePoint
I follow this simple rule: don't use punctiation other than underscores in filenames, and don't put an underscore first.
Encoding of attachments may also be an issue. Testing on our SharePoint 2007 site indicates that attachments in the AppleDouble format (often the default for email clients on the Macintosh) cause the email to fail silently. Be sure to "encode for Windows" if sending from a Mac email client. [Note that GMail uses MIME/Base-64 encoding, which SharePoint accepts.]