We do not keep full emails except emails in the quarantine folders (Spam and Quarantine Incident), just the metadata for the Message Audit Logs, but the quarantine is not included in a diagnostic package.
There is IP information included when the --config option is selected, it will include the XML files that your configuration is saved in. There will be maillogs showing IPs where mail was delivered.
No credentials of any kind should be included, unless the password is hashed (such as when being stored in the MariaDB database)