The main difference between a mail and a mms is that a mms, like a sms, is instantenaously received by the recipient, and the sender knows that the MMS has been sended to the recipient.

When you send a mail to someone, if this person is not in front of its computer or has not a mobile phone with mail always on (and even so, this person can check its mail once every hour), it can take a long delay between the time you sent your mail and the time the recipient get the mail.