Here are some things to try:
Messages sent using e3 Delivery Service are sent much faster than messages sent using the SMTP or Direct methods. This is because our servers do the work of merging the messages and delivering email. Upgrade to Direct Mail Pro to enjoy this and other benefits of e3 Delivery Service (like more reliable delivery).
Check the size of your message. You can do this by looking at the status bar at the bottom of the Message window. Larger messages take longer to send.
Avoid Direct delivery. Direct delivery takes a much longer time than SMTP or e3 Delivery Service. This is because your computer has to individually contact the mail servers for each one of your recipients. This can be a very lengthy processs.