Do you keep your old e-mails?

[Tactical Genius]

It really doesn't work that way, especially how EO is architected. Not sure how far I can go into it (Microsoft employee) but your Exchange data is replicated in a minimum of four different places - not just for resilience, but down to access points and other stuff. Once you get to the scale of Office 365, then actual physical costs become much less of an issue. For the SaaS stuff, there isn't really such a thing as "cheaper" storage in the datacentre, just "less immediately accessible".

The Architecture of Exchange online and how the databases and datacentres are structured isn't a secret Snake. Microsoft have been pretty transparent as to how it is all setup (to inspire confidence in customers). It uses the principles of the Exchange Preferred Architecture which is how the recommend the Exchange on premise software is deployed (essentially the same product).
There is usually a presentation every year on this during the Microsoft Ignite conference.
This is an old presentation.
https://channel9.msdn.com/Events/Ignite/2015/BRK3186

W1F raises a good point regarding the archive mailbox on cheaper/slower storage. Although Microsoft do not do this in Exchange Online, the ability to do this was one of the two reasons Microsoft rolled out this feature (with Exchange 2010). The second reason was keeping the offline mailbox cache small on your outlook profile. Back in the day, this was causing serious issues and slowing down desktop/laptops. This isn't an issue nowadays as you can adjust this size in the later versions of the Outlook client (starting with Outlook 2013).

Regarding breaking the email system W1F. There are limits to the number of Outlook rules, folders and folder items. These limits vary depending on the version you have (these limits rise with later versions). This should not break it for anyone else, but will cause issue if they are trying to migrate your mailbox to a newer/later system.

[Snake Plissken]

TG - the reason I'm being careful is that I've been reading a lot of MS documentation for the past month and losing track of which is internal, which is public and which was internal at the time of presentation but is public now. I'm in O365 FastTrack, so by necessity I'm a jack of all trades and master of none and not going to be diving into an on-premises setup.

[Tactical Genius]

TG - the reason I'm being careful is that I've been reading a lot of MS documentation for the past month and losing track of which is internal, which is public and which was internal at the time of presentation but is public now. I'm in O365 FastTrack, so by necessity I'm a jack of all trades and master of none and not going to be diving into an on-premises setup.

Yep, best to be coy if you are not sure. My relationship with Microsoft is a lot different so when they tell me stuff, they are pretty clear what is public knowledge, NDA and what should be kept quite until is made public here https://techcommunity.microsoft.com/.../bg-p/Exchange
Enjoy the FTC, not had much interaction with them as they are too bleedin strict so end up migrating mailboxes myself. And they really really love their documentation.
I know you do M365 onboarding projects, I assumed you were from a messaging background like myself?

[Snake Plissken]

No, I come at it from the SharePoint/collaboration/compliance side of things.

FastTrack is a free benefit to provide guidance not support. So we're there to go "Have you considered this? Are you aware of this? Best practice is to do this." as opposed to actually fixing problems or reviewing designs. One of the hardest things that I'm learning to do is not to commit to a yes or no answer at certain times.

[Tactical Genius]

Ahhh, that makes sense.

My engagements with the FTC have been with the free assistance they offer with moving mailboxes once the hybrid environment has been setup and configured.
However, they requests migration lists to be uploaded 2-3 weeks in advance which does not work for many customers as they are not that organised. They usually know a week at best and like to add/remove people last minute.

When you said you had a gig at MS, I assumed it was with MCS for some strange reason.