The biggest cause for delay in mail delivery is when a certain SMTP server is not reachable. For example, if you were to e-mail firstname.lastname@example.org, our SMTP server would first make sure that the domain name "domain.com" exists. If not, the mail is sent back to the sender with a "host not found" error.
Next, the e-mail is sent to the SMTP server for domain.com. If the SMTP server is unreachable, it will try again for anywhere from the next 4 hours to 5 days. This is the number one cause for e-mail delivery delays: problems with SMTP servers.
Another possible problem could be due to any routing problems between mail server and "domain.com". We assure our clients that once you have sent your outgoing mail through your SMTP server, it is automatically sent out for delivery. Beyond this point, there is nothing we can do to speed up the delivery time for e-mail.