So I've had my mail servers set up and working for a month now, and there are a few things I haven't done. My old mail server is still set to send from a domain of craig-james-stewart.co.uk by default, and it is no longer in the SPF record as a sender for that domain, so I have had to fix that so that I can continue to receive emails from it seamlessly. I've also had to alter the contact form on my website for the same reason. As well as these minor tweeks I have come to the realisation that I ignored time drift when setting up the mail servers, easily corrected by installing ntpd in it's default configuration on debian, apart from my rather strict iptables rules. So having fixed that, the only thing left to do, is configure certbot to auto-renew my ssl certificates, which is as simple as adding a couple of cron entries. So now I have two mail servers that will continue to work, with little maintenance effort. I still need to look at DKIM and DMARC, but those can wait.