To change the default font for outgoing email in Mozilla Thunderbird first go to the Tools menu and click Options:
In the Composition configuration click the general tab and then select a new font in the HTML section:
It is also a good idea to check the Send Options to verify that recipients will get plain text if they identified as not being able to receive HTML email.
In the Composition configuration click the general tab and then select a new font in the HTML section:
It is also a good idea to check the Send Options to verify that recipients will get plain text if they identified as not being able to receive HTML email.