Should IMAP/SMTP SSL ports be blocked at the firewall?

Our standard practice is to block all incoming/outgoing ports on our firewall and only open what is needed. With the proliferation of handheld devices using push email technology, it seems we are having to add more and more rules to allow SSL IMAP and SMTP traffic to outside servers.

It’s been suggested to me to open port 993 and 587 completely so visitors to our network can still receive email that is pushed to their devices without us having to put a rule in.

What are other folks doing with this? Anyone think this would be a huge security hole?

Thanks!
Bryan