While on vacation recently, I found myself unable to send email while at a wireless hotspot, I couldn't connect to my usual ISP's outbound mail server, and the hotspot wasn't providing this access either. I never actually send mail through my own domain, and with my access restrictions, was unable to do so from a foreign ISP.
After some digging, I installed Pop-Before-SMTP, a handy little utility that integrated with Postfix to create an access list that can be checked to allow outgoing mails to be sent. It works by creating a list of IPs that have successfully logged in to check mail (via POP, or IMAP), then allows those specific IPs to send outgoing mail, and will automatically expire those IPs in a configurable number of minutes. Also, to avoid issues with mail rejections, I configured my Postfix to relay all outgoing mails through my home ISP rather than delivering directly. It worked great the few times I needed to use it.