Version:           1.9.9 (using KDE 3.5.10)
OS:                Linux
Installed from:    Debian stable Packages

Situation: 
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- Mails stored kmail in maildir format use special characters (at least comma and colon). E.g. "1120923112.4769.TEuXE:2,R"

- Current usb sticks are mostly formatted with fat/vfat file system

Problem: 
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fat/vfat isn't able to store files with at least the colon character, so you aren't able to backup kmail mails on usb stick out of the box.

Fix:
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I know that problem (or at least the restriction) concerns to the old Windows (v)fat format, there would be no problem if usb sticks would use a better file system. I know also that one can work around it (pack mails into tar file, ...).

But apart from the fact that the above mentioned restriction means also compatibility (least common denominator regarding the charset): Is it really necessary to use these special chars in the file names? User don't have to deal with these files directly, so readability isn't necessary. On the other side software doesn't (at least in principle) matter wether there's a colon or an "x", "y" or "z".

Or is it a requirement of the maildir format?

Suggestion: Avoid special chars in filenames if it isn't necessary