For Microsoft 365.

* For Google Apps please refer to here

Question:

Mail Archive (Microsoft 365) 保存的电子邮件有哪些？

Answer:

Basically, all the incoming / outgoing mails are the targets.

Target

- Internal corporate network -> Internal corporate network Outgoing mail
- Internal corporate network -> External network Outgoing mail
- External -> Internal corporate network Incoming mail
- Internal corporate network -> Mailing list Before the deployment
- External network -> Mailing list Before the deployment
- Automatically transferred mail

Exceptional cases

- Internal corporate network -> Internal corporate network Incoming mail (Since it gets archived through the transmission pathway)
- Internal corporate network -> Mailing list After the deployment
- External network -> Mailing list After the deployment
- The incoming mail rejected by Exchange Online

NOTE: It is probable that it may change according to the Microsoft 365 specification modification.

Regarding the other mails, it depends on Microsoft 365 specification / settings.
For further details, please check with Microsoft 365 resellers.