Text encoding issues on iPad Mail

Question

Character encoding

Every day I send myself some notifications from the site. And for some reason, in some letters, Russian letters are always displayed in the header of the letter with incomprehensible characters. Moreover, it happens that the beginning of the heading is correct and incomprehensible characters begin from the middle of the word, or vice versa, the end is normally displayed. If you look at the mail through the browser, then it is normally displayed there.
How can I send letters so that this problem does not exist? There is full access at the code level when sending an email.

Answer 1

Question removed. It turns out that the encoding must be set not only for the body of the letter, but separately for the header. And if the encoding is explicitly specified, then the problem disappears.

Answer 2

As far as I know, there should be no problems when sending emails in Unicode. If there are, then the apple mail client is crookedly configured. Checked on multiple mailing lists.