Mail Server IP address

The activation email you would have received from the hoster would contain that information. It's usually something like mail.domainname.com.

You can open up a command prompt and try the following:

C:\>nslookup
Default Server:  mydns.local
Address:  10.0.1.2

> server 8.8.8.8
Default Server:  google-public-dns-a.google.com
Address:  8.8.8.8

> set type=all
> gmail.com
Server:  google-public-dns-a.google.com
Address:  8.8.8.8

Non-authoritative answer:
gmail.com       internet address = 172.217.8.165
gmail.com       AAAA IPv6 address = 2607:f8b0:4009:803::2005
gmail.com       nameserver = ns3.google.com
gmail.com       MX preference = 40, mail exchanger = alt4.gmail-smtp-in.l.google
.com
gmail.com       nameserver = ns2.google.com
gmail.com       MX preference = 20, mail exchanger = alt2.gmail-smtp-in.l.google
.com
gmail.com       MX preference = 30, mail exchanger = alt3.gmail-smtp-in.l.google
.com
gmail.com
        primary name server = ns3.google.com
        responsible mail addr = dns-admin.google.com
        serial  = 152385824
        refresh = 900 (15 mins)
        retry   = 900 (15 mins)
        expire  = 1800 (30 mins)
        default TTL = 60 (1 min)
gmail.com       nameserver = ns1.google.com
gmail.com       nameserver = ns4.google.com
gmail.com       MX preference = 5, mail exchanger = gmail-smtp-in.l.google.com
gmail.com       text =

        "v=spf1 redirect=_spf.google.com"
gmail.com       MX preference = 10, mail exchanger = alt1.gmail-smtp-in.l.google
.com