feat: more mail sending backends and plugability (#9608)

* [FR] Improve Email handeling
Fixes #7950

* extend implementation of email thread and message models

* add missing args

* add unit test

* increase test coverage

* make key not necessary

* do not consider in coverage

* add email apis

* Add email admin

* fix email configuration check

* improve rendering

* squash migrations

* add config value overview

* log if mails were send

* add additional headers

* fix api unit test

* fix url resolving

* add InvenTree specific task to issue mails
required to extend sending options (prio, reply to)

* use internal sending task to keep telemetry cleaner

* add prio handling

* add plugin handling

* add setting

* factor plugin method out

* add typing

* move function

* bump version

* fix import path

* add a test for the test endpoint

* fix checking logic

* Add anymail sending / tracking handling

* add more ordering fields to api

* remove unneeded assingment

* add basic docs

* handle incoming emails with anymail

* Add inbox handling
Closes https://github.com/inventree/InvenTree/issues/7951

* add list of supported ESPs

* add better error transparency when sending fails

* add missing migration

* combine migrations back down

* fix todos

* fix qc export

* fix missing model props

* add tests

* ensure things are passed as a list

* fix list formatting

* fix deps

* move tests

* add testing with anymail

* allow handling of priority and headers

* add test for events

* add test for inbound messages

* rename variable

* increase coverage

* fix format

* add setting doc

* fix link

* rename fnc

* disable pro test

* make messages clearer

* fix doc syntax

* fix assign

* fix test

* revert test disablement

* add enum

* disable check for now

* try changing test around

* add incoming mail processing

* fix import

* add docs

* Fix mail.md

* bump deps

* fix api version

docs/docs/settings/email.md

@@ -8,5 +8,34 @@ InvenTree can be configured to send emails to users, for various purposes.
 
 To enable this, email configuration settings must be supplied to the InvenTree [configuration options](../start/config.md#email-settings).
 
-!!! info "Password Reset"
-    The *Password Reset* functionality requires the email backend to be correctly configured.
+!!! info "Functionality might be degraded"
+    Multiple functions of InvenTree require functioning email delivery, including *Password Reset*, *Notififications*, *Update Infos*
+
+### Outgoing
+
+Mail can be delivered through various ESPs and SMTP. You can only configure one delivery method at a time.
+
+### Incoming
+
+Mail can be received though various ESPs, POP3 and IMAP.
+
+When using POP3/IMAP InvenTree removes email that were processed. This is to prevent duplicate processing of the same email. You can specify a archive folder, that mails should be moved to after processing. This is useful for retaining manual access.
+
+### Supported ESPs
+
+InvenTree uses django-anymail to support various ESPs. A full list of supported ESPs can be found in [their docs](https://anymail.dev/en/stable/esps/).
+
+Most popular providers are supported:
+
+- Amazon SES
+- Brevo (EU)
+- Postal (Self hosted)
+- Mailgun
+- Postmark
+- SendGrid
+
+### Logging / Admin Insights
+
+Superusers can view the email log in the [Admin Center](./admin.md#admin-center). This is useful for debugging and tracking email delivery / receipt.
+
+{{ image("admin/email_settings.png", "Email Control Pane") }}