I have observed this issue when I changed my Signature.

I wanted to take Tahoma font throughout the Body which was earlier defaulted to Calibri.

SO I put term "Thanks" with the required font and colour.

When I was writing emails then I used to come on this Term and then from here I used to write the emails. So whenever I used right click in the email body then it was also directing signature: rightly so.

So now I have updated the font for the Body from other available options
Insert>Signature>Personal Stationary.

And now I am able to get the required font and on right click I am getting usual window of Cut, Copy, etc.

Check if this is your problem. It can be resolved.

Cheers,
Mandar