6 files declare their use of MailFormatHelper in Drupal 10

ContactSitewideTest.php in core/modules/contact/tests/src/Functional/ContactSitewideTest.php

HtmlToTextTest.php in core/modules/system/tests/src/Functional/Mail/HtmlToTextTest.php

MailFormatHelperTest.php in core/tests/Drupal/Tests/Core/Mail/MailFormatHelperTest.php

MailTest.php in core/modules/system/tests/src/Kernel/Mail/MailTest.php

PhpMail.php in core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php

TestHtmlMailCollector.php in core/modules/system/tests/modules/mail_html_test/src/Plugin/Mail/TestHtmlMailCollector.php