Improve unsubscription, bounce handling & their integration

Back to this topic in 8.x.
The module is not popular although it's a very important issue with mail reputation.

There's more complexity we need to cover here.
First, unsubscribe as in stop sending mail is a status that can apply to users only (without simplenews) and subscribers only (that are no users yet).
As such, the unsubscribe status should be a field.

Our thoughts have been more towards bounce processing states as in "Status of mail address".

• Send
  • Still send, N bounced happened

• Do not send
  • Suspend sending, user requested
  • Over quota
  • DNS not resolving
  • DNS not resolving
  • No mail server on target domain
  • Delivery rejected
  • Too many bounces
  • Bounced as SPAM

Till now it's only a mechanism, there is no real status yet that a user can suspend sending mails.
Similar to payment module, the field "Status of mail address" can be generalised and originally can be reduced to two states "Send" or "Do not send".
A UI plugin might add the "Suspend sending, user requested" and allows this as new setting on the user profile / subscriber.
A Bounce plugin might add all the typical bounce states.
That's not enough though, since bounce processing usually involves counters. "Still send, Nth bounce happened"... with some configurable threshold to switch to stop sending.

We will need to decide if this plugin architecture is a Simplenews thing or if it's a next major release of unsubscribe or anything else like a module "mailstatus" to be released separately.

Unsubscribe has no 8.x branch yet.

And we clearly went the road down to support these concepts through inmail and mailmute in D8:
https://www.drupal.org/project/inmail
https://www.drupal.org/project/mailmute

There are separate issues about this and mainly, it is already well integrated. It's just that things can still be improved a lot until they are just perfect... ;-)

Postponed as mailmute does not yet have any releases