django-mailer: Queue stalls when non-SMTP email backend raises error

I’m using django-mailer with a django-anymail backend (specifically, MAILER_EMAIL_BACKEND = "anymail.backends.postmark.PostmarkBackend").

Anymail is not based on smtplib, so it raises its own exceptions for rejected recipients or other errors. Because django-mailer is looking for specific SMTP exceptions, an Anymail exception ends up stalling the queue, rather than deferring the unsendable message. (The exception aborts the send_mail management command, leaving the unsendable message at the top of the prioritization list for the next send.)

What would be a reasonable way to solve this? (I maintain django-anymail, btw.)

• I don’t think it makes sense to add Anymail-specific exceptions to django-mailer. (Because that starts down the slippery slope to adding specific code for all other email backends.)
• django-mailer could define and catch its own SendMailException (e.g.), in addition to the current SMTP exceptions. Then other packages that want to play nice with django-mailer could raise mailer.SendMailException when django-mailer is installed. (I’m not at all opposed to adding this to Anymail. But it would sort of start the opposite slippery slope, where email backends need to add specific code for any likely queueing wrappers.)
• django-mailer’s send_all could just catch Exception, rather than specific SMTP exceptions. I realize broad catch statements are generally a bad idea, but this seems like a case where it might be appropriate.

Thoughts? I’m happy to submit a PR if one of the django-mailer changes makes sense.