Email archiving solution

Hey, we are looking at different email archiving solutions. We are using Mailstore at some of our clients. We basically need something similar but want to go open source.
Most of our clients are using O365, we want to keep the archive local, either at the client or on our NAS at our office. Any recommendations? Thanks!

Seems like no one here is doing any email archiving?

I’m not much of a email admin but have you looked into mail piler?

Thanks I will definitely have a look