Description
In symfony/mime version 5.0.8, the file Message.php introduced a new requirement was introduced for a 'To:' field in the email message. See line 125 here. For certain email distribution lists, it is desirable in many situations to hide the recipient list by placing recipients in a 'Bcc' field instead. The 'To:' field is not necessary and should not be required.

Desired behavior
Don't require a To: field when a 'Bcc' (or 'Cc'??) field is provided.