4.4.1 Cannot connect to host - how to fix it?

Question

Mail server

Hello.
We move from one Internet provider to another. As a result, the external ip-address changes.
Old ip - xxxx
New ip - yyyy
In the DNS zone of our domain, we changed the a-record of our mail server to a new IP yyyy
But from the inside, we still continue to send via ip-address xxxx
These 2 ips are working for us. We haven't turned off the old one yet.
But after a new IP address has been registered in the DNS zone of the domain, some domains simply kick us off when sending due to:
1. cannot connect to host , or 2. possibly forged hostname for xxxx
I called the provider - they registered PTR to the new IP address. But this did not fix the issue.
What could be the problem?

Answer 1

nslookup -type=TXT mydomain.ru
will probably contain something like "v=spf1 +a +mx -all" or "v=spf1 +a +mx ~all"
add to the entry: "v=spf1 +a +mx ip4 :xxxx -all"