My work has gradually moved from Web -> Email. I learned about email while working as a sys admin for a website sending 100k mailouts and now I've become a bit of a specialist writing email apps. Postfix is my weapon of choice, you can extend it externally via its unix pipe & socket routing.
Currently I'm building an outgoing SMTP service for people network hopping - Wifi, 3g etc.
It has been interesting to see the rise in inter-SMTP server communications going SSL. Google & Hotmail, for instance, both use TLS to send their email, even when there's no requirement to do so, TLS is chosen first.
So people often trumptet BBM as end-to-end secure but if you use Google Mail with your mobile email and set it to TLS only and only send to other Google & Hotmail users then the same is true.
I've also had a report of a German ISP that deep packets SMTP traffic and changes STARTTLS to XXX "for security reasons". I suspect that they have a tap in front of their closed source MTA to log the traffic to satisfy the European Data Retention Directive.