imap.prontonet.net has no MX records.
Question and response
Question
dig imap.prontonet.net. MX
Answer
The response has no answer records.
Authority
prontonet.net. 900 SOA nsb.vtx1.net. fieldopadmin.vtx1.net. 37 10800 3600 2419200 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0