First: suspend, do not delete
A suspended account stops the person signing in and keeps their mailbox exactly as it was. Everything below still works on a suspended account. Delete it first and most of it stops being possible, so suspend on day one and decide the rest calmly.
The ways to keep the mail
Data migration. The Admin console's data migration service can copy mail from the former employee's account into another account in your organisation — the successor's, or a shared one.
Vault export. If your plan includes Vault, you can export the user's mail for use outside Workspace. It is an export: it produces files, and nothing is imported anywhere for you.
Data export. An administrator can export the organisation's data. Like Vault, it produces an archive to download rather than a mailbox someone can use.
Archived User licence. Keeps the account in place, searchable and exportable, on a lower-cost licence than an active user — bought through Google's sales team or a reseller.
Deleting a user brings up an offer to transfer their Drive files to someone else. Their mail is not part of that transfer, so settle the mail before the account goes.
What none of them do
Every option above preserves the mailbox. The successor still inherits thousands of threads and no idea which three matter this week — the customer who asked a question and never heard back, the renewal promised for “end of the month”, the supplier who used to write every Friday and stopped.
That is the part Offboard does. We read the mailbox once, with your permission, and write the successor a brief: who they dealt with, what was promised in each direction, what falls due in the next two weeks, and what is waiting on a reply. Every line links to the message it came from. If you want the mail itself in the successor's inbox as well, it is copied under one label of its own.
Related: Suspend, archive or delete: what happens to a former employee's mailbox · Writing a handover when the person has already left