Our iPhone app, MailTracker uses secure 128-bit or 256-bit encrypted HTTPS communications with our backend. On top of that, we also encrypt certain data even within the HTTPS channel.
Our mail sending services (SMTP) communicates over secure, encrypted channels only. We only use TLS (port 587) or SSL (port 465) when connecting to your email service provider and when communicating with your iPhone. Basically we have end-to-end encryption on all data communications.