Hi,
>>How do I make sure that the emails being delivered by them still show who sent the email in the header?
Generally, if you set the Send on Behalf permission to shared mailbox, there would be a
Sender field in message header to indicate the true sender of this message. For example:
The following is a message send by Steve Fan on behalf of Winni Liang, in message header, the From field is Winni Liang while the Sender field is Steve Fan:

In your scenario, please check your send on behalf of permission to make sure it is not the Send As permission which there is no information about the true sender in message header.
Best Regards,
Winnie Liang
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