This was driving me crazy but appareantly it's very simple. I created a rule with the rules module which would send a mail to multiple email addresses. Making the rule send emails to one email address worked fine but when I entered more than one email address the whole process would grind to a screeching halt and no mail was sent.

It appears you have to use a specified way to enter multiple email addresses. Above the field where you have to enter these it states: "The formatting of this string must comply with RFC 2822." But what is "RFC 2822"? That's quite vague. Searching for RFC 2822 turns up a Wikipedia entry. See the field in the screenshot below:

After a lot of trial and error I finally figured out that the correct "RFC 2822" way to enter email addresses is this: <emailadress@one.com>,<emailadress@two.com>. As you can see in the screenshot below:

Easy right?

Drupal 7RulesRFC 2822Email