Our organization has blocked all outbound SMTP traffic under the assumption that we are preventing potential botnet spamming issues which lead to blacklisted IPs. However, I've seen other organizations which have been set up to allow this and am curious what recommendations are for being the most secure and protected in regards to SMTP traffic.
An initial thought is that perhaps just unblock 465 and 587 so that only the secure channels are allowed (I don't believe you need 25 open for the secure SMTP ports to be used). But thinking further, perhaps find a list of common SMTP servers of large ISPs that would be more-or-less considered safer than (say) smtp.spammersite.ru?
Any recommendations on this?