Font size changing unexpectedly in Outlook 2003

Hi,

First time I have posted, so please be gentle with me! I have a user who, when she sends an email, is experiencing a problem with the font changing size across part of email, the problem is, this appears to be happening AFTER she sends the mail, so when she checks the message, all is fine, but when she reads the message in sent items, it has changed. Have also confirmed that the recipient copy has changed.. Very strange.. Any tips and advice welcomed..