Thoughts, tutorials, and insights from the trenches.
'Zero downtime migration' sounds like it needs special tooling. Almost always, it just needs the new server fully tested before DNS changes, and a TTL that was lowered days in advance.
Every DNS zone I inherit has a handful of records nobody can explain and is missing one or two that actually matter. Here is the short list I check first.
The mail server software is the absolute easy part. The reason self-hosted email ends up in spam folders almost always comes down to four specific DNS records and a sending reputation that takes actual time to build.