What encoding to write in html letter?

Question

Email

Randomly saw this situation.
The client received a letter to Yandex (I don’t know how to open it) part of the encoding is normal, part is hieroglyphs.
Substituted variables are especially incorrectly displayed, they are loaded at the subscriber in utf8.
part of the letter: For "AKULA, Ltd.", we offer
part of the letter
: source (watched through online decoders) - all texts are defined as utf8
The difference between these texts: they could be collected in different editors.
how to recode an already prepared letter?
How to optimally make the encoding, what would be displayed the same for everyone and accepted by mailers?