Which mail server is best…

Hi,
Our company is about to set up 40 branches around the uk and all these staff will require email facilities, as well as 50 LAN users. These 40 branches will all have individual domain names making a total of 41, most will have dynamic IPs and all will need to send/receive mail. I also would like the ability to monitor the mail, apply rulesets to mail, and virus check mail for compliance purposes. Is MS Exchange the kiddie for this or is there another product out there which will do the job.
Any help appreciated & thanx for your time..