spf.alumni.cuhk.net has no MX records.
Question and response
Question
dig spf.alumni.cuhk.net. MX
Answer
The response has no answer records.
Authority
alumni.cuhk.net. 600 SOA ddi-gm1.itsc.cuhk.edu.hk. hostmaster.cuhk.edu.hk. 201701284 1800 900 1209600 600
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0