those are not comments. According to RFC 2822 CFWS syntax, comments would be enclosed in parentheses, which is not the case here. Message with comments would be parsed without issues:
Date: 20 Apr 2022 10:03:29 +0200
Indenting a line actually makes it a continuation of the previous line - therefore, the indented 'To' and 'From' lines are actually part of the 'Date' header, so equivalent to this:
Date: 20 Apr 2022 10:03:29 +0200 To: firstname.lastname@example.org From: <email@example.com>
Those strings are not comments, they are a part of the Data header, and that's why our parser fails.
Outlook, Windows Mail and Thunderbird are notably very benevolent about what they parse - for example, when I download a PNG image and save it as 'message.eml', Outlook does not complain at all and pretends it's a message with a body text of "Ê#d|eftxè‘oùÓyÓ! I¼" - we don't think emulating that in our mail library would be a good idea.
However, you can make our parser more benevolent by enabling
MailMessage.Settings.IgnoreUnparsableHeaders option - this will result in the parser ignoring the wrong 'Date' header (along with the emvedded 'To' and 'From' strings).
Unlike Rebex Mail with
IgnoreUnparsableHeaders, Outlook's 'Date' header parser actually discards the extra text and parses the beginning. We'll try making our 'Date' parser a bit more benevolent as well for the next release.