Exchange 2003 to 2007: the size limit snafu

Here’s something to be aware of when installing Exchange 2007 into an existing Exchange 2003 environment: once you’ve run /prepareAD /prepareSchema etc., if you previously had no global message size limits,
Exchange 2007 will impose a 10MB size limit on all messages. Even before installing the first server role, the schema & AD updates do this.

Yes, it is documented in a KB http://technet.microsoft.com/en-us/library/bb310771.aspx but it took me by surprise because it’s not mentioned in any of the "upgrading to Exchange 2007" planning docs. I followed all of the install instructions carefully, but when users started complaining about messages with large file attachments getting rejected, we knew that the AD preparations had done something bad.

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A quick search led me to the above referenced article, and we changed the global limits back to "unlimited". It took about 15 minutes for the change to propagate to all users & very little harm was done.

But I think that this should be included in everyone’s planning checklist:

Find out if there are currently global limits on message size:

• if yes, then don’t worry – it won’t overwrite existing limits
  
• If no, then you will need to quickly reset them in 2k3 after prepping the AD