DNS and domains
SRV record
A DNS record that names the host and port for a service, so a client can find it without the user typing server settings.
An SRV record answers "where is service X for this domain". The name encodes the service and protocol, _imaps._tcp.example.lu, and the record gives a priority, a weight, a port and a target host: 0 1 993 imap.example.lu..
Mail clients use them (RFC 6186) to find the IMAP and submission servers from nothing but an e-mail address. Calendar and contacts clients do the same with _caldavs._tcp and _carddavs._tcp; SIP phones, XMPP and Minecraft servers use them too.
A target of . means the service is explicitly not offered, which is a cleaner answer than no record at all. The target must be a hostname with an address record, not a CNAME.