The OSX firewall is switched off.
Any attempt to connect to any SMTP server, from any network, times out.
So whether the user is in Canada trying to use hotel broadband, or at home on their own ADSL connection, or at an airport, or a MacDonalds, the connection attempt times out.
Whether they use port 25 or the secure SMTP port (5-something?), the connection times out.
All other netty services seem to work: POP3, http, you name it.