MX stands for 'mail exchanger'.
It tells a 'sending' mail server how to find the 'receiving' mail server, given in order of priority, where the lower numbers are given higher priority. The MX records for 20i.com are 'mx.20i.com' with priority 1 and then 'mx2.20i.com' with priority 2. They are tried in that order by the sending mail server. Simply, 'mx.20i.com' and 'mx2.20i.com' are regular A records that point to the IP address of each mailserver that can handle mail for 20i.com.
