Ah hell, I'm a FAT32. So there's no way for me to put anything over 4 megs onto the USB?
Umm, that's 4 gigs, but "No."
However: Back in the days when we carried data around on flexible pieces of plastic inside fiber sleeves, the size limit of the medium was about 1.44 Megabytes. Not Gigabytes, like your flash drive, but Megabytes. These fragile, primitive devices were called "Floppy Disks," although if one was floppy it was probably too damaged to use.
To transfer files larger than this limit, primitive man first developed compression software. To transfer files whose compressed size was larger than this limit, a great programmer named Ugh (I don't know why I'm doing this stupid olden-days cave-man riff; please excuse me) came up with the "spanned file." The compressed file was broken into 1.44 MB chunks, which could then be re-assembled by the decompression software at the receiving side.
So either reformat your USB key with the NTFS filesystem, or re-compress your mail file but find the settings in your compression software to break it into multiple files of less than 4 GB each. How to do this depends on which software and which version of it you are using.
Edit: Now that I think about it, 1.44 MB floppies are fourth generation or so. I have some 360KB 5.25" floppies around somewhere, although my drive requires a floppy controller, and those don't seem to exist anymore. The 8" floppy was in use at about 180 KB in 1972, if I can believe what I read on Wikipedia.
And the "fiber sleeves" were for the 5.25" floppies; 3.5" floppies have rigid cases. Old Grogg here is getting mixed up about his earlier days.
Edit: I'm going to go take my nap. I originally wrote "NFS filesystem" instead of "NTFS filesystem." NFS filesystems are for mounting disks on remote Unix hosts.