zero.apple.relay.mailchannels.net has no SRV records.
Question and response
Question
dig zero.apple.relay.mailchannels.net. SRV
Answer
The response has no answer records.
Authority
apple.relay.mailchannels.net. 900 SOA ns-1660.awsdns-15.co.uk. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0