Redirect email from in-house Exchange 2003 to hosted Exchange 2013

I've done something similar before but not exactly the same, so want to sanity check it first.

If I create a new SMTP connector in ESM2003 with the address space of the local domain, select to allow messages to relay to these domains and forward all mail through the connector to the hosted Exchange service - will this redirect all incoming email to the new server while public DNS is propagating instead of delivering to the local mailboxes?
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devon-ladAsked:
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Simon Butler (Sembee)ConsultantCommented:
No.
Exchange will recognise that the email address is local.
DNS propagation is a myth, as DNS changes are live almost immediately. It is caching that you have to worry about.

Instead, make the change in DNS and then block port 25. Any sites that have cached the information will then be forced to do a fresh lookup and get the new address.

If you really want to catch everything, then stand-up an IIS SMTP server to relay the email to the new address.

Simon.
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devon-ladAuthor Commented:
Ah, so if an incoming email is in a queue somewhere because port 25 had been blocked - the sending server does a fresh DNS lookup each time it retries to send?  I never realised this.
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Simon Butler (Sembee)ConsultantCommented:
Not every time, but it will do so quicker. It does depend on the behaviour of the individual MTA. It can help if you turn the TTL time down in advance (so if the TTL is two days, turn it down to 1 hour at least two days before the changes). Then any modifications are seen by the internet quicker because they are caching the information for a shorter time period.

Simon.
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devon-ladAuthor Commented:
Ok thanks - will probably go with the standalone SMTP relay - this is how I'd set it up in the past.  They have a Hyper-V environment so easy to spin up a new server.
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