When you switch nameservers, it can take up to 24 hours for the nameservers to take effect. The MX records will be determined by where the nameservers point. The Nameservers point the domain to a specific server. When the nameservers resolve, it loads the DNS zone file where the nameservers are pointing.
So if you have MX records set on one server and your domain points to that server, those MX records will work. If you change the nameservers to another server, the DNS zone file that on the new server will load.
Where the nameservers point will load that dns zone. Now if you change the nameservers, its possible the email routing could have been messed up for the domain. Check in your cPanel > mail > MX entry. Select your domain from the list and check the routing. If you want to deliver your email to our server, make sure it is set to local. Here are some links with more information on this.
How to Modify your MX Records
550 no such user errors are common to the email routing being set to automatically detect. When changing nameservers around, the server can get confused on where to route the email.
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