Currently there are 2 problems with alerter:
1. For every alert it opens new connection. That means establishing new TCP session, SMTP negotiation (EHLO/HELO). For large volumes of emails that means lots of unnecessary actions. Many smtp servers allow to establish connection, issue EHLO/HELO and then generate more than one message thus reducing overhead (attached log of sending 2 messages over one session)

2. For every alert Zabbix generates separate message, while it can generate 1 message with lots of emails (I've seen such requests on forum). For example, we have 26 users in the group, they all got notified about problems in host group. If there are some serious problems in this group we got LOTS of emails. If the server uses public email server (gmail, aol, etc) for sending alerts it would be sooner or later banned for high volume of emails.
Zabbix has two variants for sending messages on trigger actions:
1) send message to specific user
2) send message to group of users
In the 1st variant there's no need to try to aggregate emails, Zabbix admin should think of it by himself
In the 2nd variant Zabbix do unnecessary and strange things - it parses group and creates separate notifications for every user. I guess this was done for unification of notification process, but I think it should be
But it could create one record in the alerts table: "notify group 7" and then alerter could do smth like: "select group_concat(sendto) from media where mediatypeid in (select mediatypeid from media_type where type=0) and userid in (select userid from users_groups where usrgrpid=7) group by mediatypeid;" (just plain dumb request, I think it could be written much better way). After this request we got list of recipients suitable for "To:" field in email.