I don't use Evolution, but I suspect being a Gnome application that it works like
web browsers, where fonts can be enlarged using Ctrl-+ as many times as it takes
to grow the fonts adequately. Possibly it also has a minimum displayed text size
option as web browsers offer.
Ctrl-+ works on the entire window. So if the tiniest font is enlarged enough to be readable, the rest of the message doesn't fit anymore. And Evolution remembers it, so you have to be careful to count the number of times you do it so you can get back to normal without a lot more experimenting.
It's not my responsibility to deal with messages the senders aren't serious about being read.