We have had problems before with SMTP port 25 (smtp) - some PC's were sending spam mail throught that port.
So we blocked that and only allowed only outgoing smtp servers we needed.
Howerver, i read there is 587 port (smtp mail submission) which, for example, is used by gmail - i`v unblocked some common ports like http, https, ftp, ssh etc - is it safe to unblock 587 port to any destination ip address? Is/can spam be sent through that port like it was through 25?