Exchange 2010 SP1 to SP3 Upgrade? Will Exchange Function with Mismatched Versions?

We're currently working on upgrading our Exchange 2010 servers from SP1 to SP3. I have read on Microsoft's site to start with the CAS/TSP servers first and then once those are up and running, upgrade the MBX servers.

My question is, if we run out of time during a maintenance window and we cannot get to the MBX servers until the next maintenance window (24 hours later), will we run into issues with the CAS/TSP servers running SP3 while the MBX servers are running SP1? Or will they function fine until the MBX servers are upgraded?
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Will SzymkowskiSenior Solution ArchitectCommented:
It is a best practice to have ALL of your servers on the same SP version and RU. However they are backward compatible so they will operate fine. The biggest thing is making sure that all of your CAS servers are on the same SP version as you have stated. If you cannot get to the mailbox servers within the time frame for maintenance, Exchange will continue to operate normally. So you will be fine if you cannot get to the mailbox servers until the next maintenance window.


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Marwan OsmanCommented:
it is not a problem but the best practice is to upgrade them soon:

The upgrade process is simple and can be summarized in a few steps as described below. Bear in mind that a Database can be moved among servers running different Service Pack versions but it is good practice to keep all servers running the same version sooner rather than later.
AmitIT ArchitectCommented:
Marwan OsmanCommented:
Sorry Mr. Will, your comment appeared only when I have submitted my comment.
danherbonAuthor Commented:
Thank you for the initial confirmation and then the link to the step by step guide.
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