Hi @bereket solomon ,
Welcome to post in Microsoft Q&A forum.
so my question is can I use this single fortimail appliance for two mailboxes as a replacement for the edge?
For the Exchange Edge server role, it's supported to deploy a single Edge server for an AD site which includes multiple mailbox servers. So if the device you mentioned is equal to Edge at this point, you should be do so as well.
However, as here we we mainly focus on general questions about Exchange server and I myself am not so familar with the fortimail device, so actually little we can do to help on your specific concern involving the fortimail device. Given this, personally I'd recommend referring to your Fortimail documentation or contact their support for further confirmation. Thanks for your understanding.
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