In the previous section, we learned to handle inbound e-mails. Now, it's time to learn about outbound e-mails, which can either be basic text based messages or rich HTML based messages (with or without attachments), and sending them through Apex. The Force.com platform allows us to send e-mails by using a broad set of built-in classes and methods. The outbound e-mails can be sent by a trigger, Visualforce controller, e-mail services or by any other platform.
There are two types of e-mails that can be sent by the Force.com platform.
Single e-mails: These are e-mails with the same e-mail body and they are sent to one or more e-mail addresses.
Mass e-mails: These are the e-mails with the personalized message bodies that may be sent to a large number of addresses. However, currently, this feature has an upper limit, and the different versions of this platform have different limits, such as Professional Edition(250), Enterprise Edition(500), and Unlimited and Performance Edition...