I'm surprised that this is still not possible (as far as I can tell) in Airmail 3. Conversation with support team also says this is not possible.

All plain text emails are rendered in pre-set font which is not fixed width, making tabular data very hard to read.

What is strange is that the feature to set the plain-text font when COMPOSING an email is there - so clearly the requirement is understood (unless this setting section was just copied direct from Outlook...).

Even Outlook (for as long as I can remember) and on OS-X has allowed to choose the font for plain-text email display, and specifically allow a fixed-width one.

The way that this is set in Outlook is to set the plain-text email composing font, which also affects the way the emails are displayed.
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Hopefully a similar change from Airmail would be simple to implement.

That would be extremely useful - if not necessary - for pre-formatted text mails (like the ones logwatch sends)!

seriously.
this has always been a huge issue with airmail. i have flirted with it off and on since the 1.x days.
i'd like to take it a step further in that if the email is received as plain text -- the composer should act as a plain-text composer (with appropriate fonts).

if this would be added, i'd completely move away from mac mail.

Same here. Would be greatly appreciated.

Same here. I like monospaced fonts much better for system mail. In Apple Mail it was the first setting I changed.

That would be really great, especially since I have many Mails from scripts and cronjobs, which are much harder to read with a non monospaced font.

Could this be implemented with a theme variant?

Definitely a life-saver