Originally Posted by Perfect Storm
Those that are instant have paid/worked with RIM to make them so.
Others are not instant to prevent server hammering and congestion
correct. certain email servers have agreements with RIM to provide instant delivery (Gmail, Yahoo ), where as all other services are accessed every 10-15 mins. if you receive lots of emails, RIM's servers know to check more often and it will check every 5 mins or so. if you think this is a pain and you need instant delivery, you might want to consider creating a separate Gmail and forwarding the email from those other accounts to that account, or have Gmail use it's POP3 retrieval to get email from those accounts.
the delay isn't much of a problem for me because I receive SO many emails a day from my college IMAP account, that it's practically instant delivery (a few mins)