Here’s a screenshot of two (spam) mails inside K-9 Mail:
One of them has little arrows/guillemets/chevrons before the second preview line, see red arrow. (In this case i have Correspondent above subject enabled, so it’s the subject line.)
If your account’s email address can be found in the To header » will be used. If your email address can be found in the Cc header › will be used. If your email address can’t be found in either of those headers no prefix will be used. That’s often the case for mailing lists.
Is it possible to disable this feature altogether? I find it cluttering and it doesn’t provide me much value. I’ve never used any other email client with this feature so it would seem strange if it could not be disabled.