Is it okay to use the word "letter" instead of "e-mail" regarding to e-mail correspondence?

Could you please advise is it okay to use the word "letter" instead of "e-mail" regarding to e-mail correspondence?

For example, is it okay to use the word "letter" in the following phrase if we are talking about e-mail correspondence: "thank you for your letter".

Best wishes, Anton

It is best to reserve the term "letter" for actual paper letters sent by "snail mail" (post).

Call an instance of email a "message". (If you simply say "your email" you are not specifying which message; there may have been several.) "Thank you for your message {sent/which I received} on Friday."

Or mangle the language and pretend it's AN email. (The majority of Americans do, though they don't call a letter "a mail".)

"thank you for your latest email" (actually you have no idea whether it was her latest email; she might have sent and received several other messages since sending that message to you)