Distorted fonts inside mails

[HervE]

Hello,

I use OTRS to answer to customers with no specific font inside my mails (although using rich text editor).

Sent emails are OK, displayed with the default font (no serif) - looks like the font of this post, by the way.

But sometimes (not always), when customers reply, I can see my previous mail at the bottom of his answer with a distorted font (smaller and serif, not beautiful and not easy to read).

Why?

Regards,
HervE