dns routing MX record

Question

I want to move an old website from a client's office server to a new hosting account but I need to leave the email accounts on my client's office server. I have done this once or twice before and what I did was to change the domains nameservers to point to the new hosting account and then in the clients new hosting account I setup a new MX record to point back to their office hosting. The problem is that this there is a lot more emphasis on email and also I may have to do the transfer midweek and possibly in the middle of the day so they don't want any real email downtime. I have 3 questions

1. If something went wrong and I needed to switch back might it take a long time for the nameserver to propagate back. I guess the answer here is yes so if that was the case then would it be that the emails sent to the domain during that time would be lost.
2. Could I test that the new MX record on the new hosting was working properly before I switched the nameserver over.
3. If everything was setup completely right might I still end up with downtime.
4. If I did an MX lookup now would the IP that I find be the one I would use for the new MX record on the new hosting.

Answer 1

My vision of situation may be wrong, so - fix dirty assumption, if it needed.

I want to move an old website from a client's office server to a new hosting account but I need to leave the email accounts on my client's office server.

But you say nothing about "where DNS-servers, which serve this domain, placed now and there they will be", but this is most important part.

• If authoritative-servers not changed, you have to do only one thing: change IN A|IN CNAME record, pointing to site. After TTL-expiration (of probably cached data) old data will be replaced by new on re-request, new requests will return new data immediatelly after zone-reload

• If you have to change DNS-servers for client's domain, you still can eliminate almost any downtime with some easy tricks

  • copy old zone-definition to new primary-ns 'as-is', edit on this (still unknown to Net as dns-for-your-zone) server NS records (they must reflect ns-changes planned), SOA of zone and www record for new website location, leave MX untouched
  • verify propagation of edited zone zone from new-primary to new-secondary|secondaries
  • when above condition executed, change glue-records for domain in registrar data
  • don't worry about MX at all: this data is the same in old and new zone-definition
  • periodically check SOA|NS data for domain in order to detect visibility on new data (/flushdns cache of local resolver for clean results)

Just thinking and side notes

• Newer use (at least avoid usage) DNS-servers of Web-hoster: changing hosting will require changing DNS and this is source of headache. Independent reliable permanent DNS-hosting for domain is good from any side
• Conside using backup MX-servers from business-critical email conditions. In this case email-lost become impossible (well, almost...) and primary MX-downtime will be invisible to external world