We are planning to upgrade our exchange server from 2007 to 2013 and I have been asked whether its possible to generate a message to all external emails that may email at this time an automated message stating that the server is currently unavailable.  I understand that there is downtime so therefore unlikely.

• Moved by Jason Johnston [MSFT]Microsoft employee 17 hours 27 minutes ago Not a development question.

There is no "downtime" as such, maybe few minutes. But not to the extend where you have to send an automated email to external senders. Most of the "flipping" should be done out of hours anyway.

You can use transport rules if you really want to do it. 

Check https://technet.microsoft.com/en-us/library/bb421708(v=exchg.80).aspx

Sorry forgot to mention we are also virtualising the new server so will have a downtime but thanks will take a look at the link you have provided.  I was also thinking of perhaps being able to edit the standard bounced back message that users get when there is an issue with email

Why would you have a downtime since you are virtualizing 2013?

Your 2007 servers should stay as is and you install new member servers (physical or virtual) and install Exchange 2013 on it, flip the services after configuring and move the mailboxes at the end after testing.

Hi em,

we recently did some 2007 to 2013 migrations, mail services ran uninterrupted as far as mailflow was concerned. There might have been a slight delay of a couple of minutes during an indiividual mailbox's move, but no user ever noticed a thing.

Physical or virtual doesn't matter either.

Seriously, you'll have enough fun with Client Access that I recommend against borrowing trouble on a front that's never occur anyway.

Cheers,
Fred

If you follow the instructions in the deployment assistant you should have no issues with downtime.  

https://technet.microsoft.com/en-us/office/dn756393.aspx?f=255&MSPPError=-2147217396

Depending on how your MX records are setup and if you have to change them, you may see some delay just due to the way DNS works on the internet.  If you have a hosted hygiene provider you shouldn't have too much to worry about.