Dkim emails sent via the Bitrix email marketing module only have problems when the site is in cp1251?

Question

1C-Bitrix

Given:

1. Mailing lists are sent from the Email Marketing module via exim

2. The dkim signature is being validated in: gmail, yandex.mail, mail.ru, yahoo mail, icloud and mail-tester.com

3. The dkim signature is NOT VALIDATED: outlook and dkimvalidator.com

4. When sending emails by email clients (eg Thunderbird), all of the above dkim signatures are seen and validated.

Conducted an investigation:

1. Googled a lot and found out that this is possible when:

a) The receiving party works strictly according to RFC
b) There are special characters in the title or subject of the letter

2. For example, dkimvalidator.com sees my newsletter like this (my site is encoded in cp1251): "Subject: =?windows-1251?B?yuvz4SDr/uHo8uXr5ekg9fD++A==?=

" the mail server and sends mailings from the site to utf-8 check whether you are validating dkim in outlook and dkimvalidator.com to understand if the encoding is the case or not.