Remove requirement to have a bounce email account setup
With some more modern SMTP providers bounce email accounts may not be necessary for CiviMail Bounces to work because e.g. with Amazon SES / Sendgrid you can get the bounces processed via webhooks instead.
However within the code base we assume we need a bounce account to generate stuff like no-reply email address or when we are getting message ids
It would be nice to get rid of this reliance on it being the default mail account which is the bounce processing one https://github.com/civicrm/civicrm-core/blob/513f6a469927c5dc5a41905feb7c3cff2cf15f0b/CRM/Core/BAO/MailSettings.php#L59
cc @JoeMurray