Thunderbird is a great choice for a mail client on Linux systems if you prefer a GUI, but I had some problems with fonts in the most recent releases. The monospace font used for plain text messages was difficult to read.
I opened Edit > Preferences > Display and clicked Advanced to the right of Fonts & Colors. The default font for monospace text was “Monospace”, and that one isn’t terribly attractive. I chose “DejaVu Sans Mono” instead, and closed the dialog boxes.
The fonts in monospace messages didn’t change. I quit Thunderbird, opened it again, and still didn’t see a change. The strange part is that a small portion of my monospaced messages were opening with the updated font while the majority were not.
I went back into Thunderbird’s preferences and took another look:
Everything was set as I expected. I started with some Google searches and stumbled upon a Mozilla Bug: Changing monospace font doesn’t affect all messages. One of the participants in the bug mentioned that any emails received without ISO-8859-1 encoding would be unaffected since Thunderbird allows you set fonts for each encoding.
I clicked the dropdown where “Latin” was selected and I selected “Other Writing Systems”. After changing the monospace font there, the changes went into effect for all of my monospaced messages!