Most of SMTP requires MX record to be pointed to a hostname instead of IP address.
For example, if you would like to setup 218.5.1.249 as the mail server of whatis.cn, you can configure in this way,
1. Resolve mail.whatis.cn to 218.5.1.249 (please fill in mail for host column, and choose A for type column, then fill in 218.5.1.249 for data column, click 'Add' button).
2. Please configure the MX record of whatis.cn as mail (leave host column, choose MX for type column, fill in mail for data column and then click 'Add' button.)
3. Configurate your email server at 218.5.1.249 to accept emails for @whatis.cn.
If you fill in mail for host column in the second step, then all the emails sent to @mail.whatis.cn (not @whatis.cn) will be received by 218.5.1.249. This may not meet with your original intention.