You need to create an MX record in the DNS zone of your domain. Something like:

@ IN MX 10 my.zome.
@ IN MX 20 server1.my.zome.

You can create 1 MX record or more. 10 and 20 server priority for input mails.

Then you need to create an spf record. There are several options here. For example

@ IN TXT “v=spf1 +a +mx -all”

allows you to send emails from A domain records, then from MX domain records and prohibits from all other hosts.

Theoretically, you can only create an SPF record with A but without MX and dont create MX DNS records. Although I have not tried this configuration.

This is the minimum set after which you will get into spam, but at least the letters will reach.

You also need to make a PTR record to avoid spam folder, but this is not possible on a dynamic IP.