This article describes the restriction on attachment names for all attachments sent through emails to create a FogBugz case.
In FogBugz, Emails can be directly turned on to Cases. So, any attachment to these emails also gets attached to respective Cases.
The restriction is, these attachments' names can't contain any special characters, like '<', '>' etc.
While this is technically impossible to occur in a Windows environment (file system would prevent that from happening), it can and does happen on a Linux based / Mac OSC environment: these OS can store filenames with '<' or '>' characters.
So, if an email is sent with attachments whose filenames contain '<' or '>' characters, and the email server passes the information "raw" to FobBugz, this can result in errors like 'Mail Import Failure'.
This can be corrected if the email is "forwarded" to the same id automatically, as this process would "wrap" the attachments of the original message and sanitizing the "rogue" characters.
Another workaround is to avoid sending attachments whose filenames contain '<' or '>' special characters. This limitation is well documented in the email standards.
Email standard documentation: RFC 2392 - Content-ID and Message-ID Uniform Resource Locators.