No immediately visible MX server. I wonder if they’re actually using an email protocol or if it’s a front end for sending log files.this doesn't seem right. your query was for mail.deepgtp.net/health, which is a url, not a domain. querying the domain gives proper mailserver data:
$ dig +short deepgtp.net mx
10 mail.deepgtp.net.
$ dig +short mail.deepgtp.net a
43.160.236.90