Returns an array of message numbers sorted by the given parameters.
Reverse is 1 for reverse-sorting.
Criteria can be one (and only one) of the following:
SORTDATE        message Date
SORTARRIVAL     arrival date
SORTFROM        mailbox in first From address
SORTSUBJECT     message Subject
SORTTO          mailbox in first To address 
SORTCC          mailbox in first cc address 
SORTSIZE        size of message in octets
      
The flags are a bitmask of one or more of the following: