sip1.linphone.org has no MX records.
Question and response
Question
dig sip1.linphone.org. MX
Answer
The response has no answer records.
Authority
linphone.org. 10800 SOA ns1.gandi.net. hostmaster.gandi.net. 1772668800 10800 3600 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0