The reasons are MANY but let's cover one simple one. Font substitution.
If you are sending HTML, Rich Text or those other than ASCII emails if there is a font they don't have, the receiver's email software and OS has to substitute a font and you see this happen.
The second reason is their email settings.
You can try using the same font in your email but given how this system works there is nothing busted here. Or you could say it's been busted for over a decade depending on your point of view.
Hope this helps,
I use Vista sp2 with IE9, McAfee Total Protection and Windows Defender. Everything is updated.
I use Yahoo Mail withinWindows Live Mail. My text size is
set at 14 everywhere.
Recipients of my email are reporting that the text size varies from very large to very small within the same message. Yet
when they forward it back (for me to see) - it looks just as I
had sent it (using text size 14).
Why is this happening and how can I fix it??
Thank you in advance for your help.