Is there a standard DNS record to indicate the IMAP server for a domain?
From a DNS perspective you have SRV DNS records which allow the use of DNS for publishing services and service discovery. Their main use is to allow services to run easily on non-standard ports and to reduce the configuration burden when setting up clients.
A SRV record has the following form:
_Service._Protocol.Name. TTL Class SRV Priority Weight Port Target
and one for IMAP is defined in RFC 6186 and would look like:
_imap._tcp.example.com. 3600 IN SRV 0 10 143 my-imap-host.example.com.
_imaps._tcp.example.com. 3600 IN SRV 0 10 995 my-imaps-host.example.com.
Most email clients don't specifically look for an IMAP server first though, but use auto discovery to derive email client settings from the email address a user enters.
If a user enters [email protected], depending on the client those typically involve either
_autodiscover._tcp.example.com.SRV record such as used by MS Exchange and Outlook
- an actual host called
- or more
A pretty good write up is found here : https://developer.mozilla.org/en-US/docs/Mozilla/Thunderbird/Autoconfiguration