Update SRV records as per RFC 8314

[Brahmasmi]

Namaste,

As per RFC 8314 [1] , the use of STARTTLS is to be deprecated in favour of Implicit TLS. Further, there is a new "_submissions" SRV record for the 465 port.

We may want to change README from

_submission._tcp.example.com	86400	IN	SRV	0 1 465 mercury.example.com
_submission._tcp.example.com	86400	IN	SRV	5 1 587	mercury.example.com

to

_submissions._tcp.example.com	86400	IN	SRV	0 1 465 mercury.example.com
_submission._tcp.example.com	86400	IN	SRV	0 0 0   .

Dhanyavaad.

[1] - https://tools.ietf.org/html/rfc8314