Email password corruption on Site transfers

Hi Folks. I just transferred 24 sites from an older version WHM to a new server using the WHM TRANSFER and SSH root login. I then spent the next 2 days fixing email passwords for people. Why would email passwords corrupt on the way over? It did seem that many of the old passwords were possibly setup a while ago, and that maybe like the "MySql encrypted passwords need to be re-saved" feature with TRANSFER, there might need to be a warning... I have another 100 odd sites to move soon. I am a little worried.