Well… as I already wrote, my home server is literally on the Internet because I rent a static public IP address from the provider.

But if you have a VPS, then you just need to do port forwarding to your server with a VPS, and then add the following entries to the mx DNS server:

you.domain.              21600   IN      MX      10 you.first.vps.
you.domain.              21600   IN      MX      20 you.second.vps.

Where 10 and 20 are the server priority Or if the VPS is part of your domain then:

you.domain.              21600   IN      MX      10 first.vps.you.domain. 
you.domain.              21600   IN      MX      20 second.vps.you.domain. 

first.vps.you.domain.             21600   IN      A       1.1.1.1
second.vps.you.domain.        21600   IN      A       2.2.2.2

And if you also have IPv6, you can do

first.vps.you.domain.             21600   IN      AAAA       fd00::1
second.vps.you.domain.        21600   IN      AAAA       fd00::2

Where 1.1.1.1, 2.2.2.2, fd00::1 and fd00::2 are the addresses of your VPS

You also need to enter the address in the SPF:

you.domain.              21600   IN      TXT     "v=spf1 +mx -all"

What does it mean

v=spf1 is the SPF version.

+mx – it is allowed to send mail from the IP addresses specified in the MX records of the domain.

-all – prohibits sending from any other servers (hard refusal).

Also, in order for the signature to work on the mail server, you need to make several TXT entries (for a detailed explanation, see my links about DKIM):

keyname.__domainkey.you.domain. TXT "v=DKIM1; ...%DKIM params%"

and

you.domain.             86400   IN      TXT     "v=DMARC1...%dmarc params%"

And you need ask you VPS provider set PTR for you VPS IP address with first.vps.you.domain. Or some providers access that config in web panel.