Mariano Gomez (The Dynamics GP Blogster) has written a great article on using Collaboration Data Objects (CDO) to send emails.
Dexterity does have the MAPI_ function library but it can only handle a single attachment.
Another method of sending email is to use the COM_ function library to communicate with the Outlook application, which will allow multiple attachments, but does require the workstation to have Outlook installed and a profile configured.
Mariano's method allows you to communicate with a SMTP (Simple Mail Transport Protocol) server directly and bypass the need for an email application on the workstation.
For more information please visit his site: Sending emails with Collaboration Data Objects (CDO) and Dexterity