Free tool
When someone types an address into Thunderbird, Outlook, Apple Mail or a phone, the app looks in DNS for the server names and ports before asking the user. This check reads the records those apps use: SRV records for IMAP and SMTP, the autoconfig and autodiscover hosts, and CalDAV/CardDAV. DNS only; the configuration files themselves are not downloaded.
What this checks
RFC 6186 SRV records (_imaps._tcp, _submissions._tcp and so on) name the host and port per service and are the method most clients other than Thunderbird and Outlook try first. Thunderbird fetches an XML file from autoconfig.domain, which you serve yourself, so you can set the display names, authentication and ports exactly. Outlook uses autodiscover, which needs an HTTPS endpoint answering Microsoft's XML; for a plain IMAP/SMTP setup it is usually left out. Publish SRV for everyone, autoconfig if you want Thunderbird perfect, autodiscover only if you run Exchange or a compatible server.
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