Backup Software That Splits Large Files For Incremental Backup

Any simple backup program that is capable of backing up large files incrementally by splitting the file and backing up only changed parts of the file? The files I'm talking about are files that get altered by appending stuff at the end of the file, e.g. log files or mailbox files, so it's generally possible to split a new version of the file at the exact point where the old version stopped, purely by file size.

Specifically, I want to back up Thunderbird and SeaMonkey mailboxes without having to create additional subfolders within those programs. I understand that if something had changed near the beginning of the file, then the whole file will have to be backed up, unless the backup program is capable of some very clever searching, but mostly the files will change only by having data appended to it.