I'm trying to mentally compare this scenario with the use of a cryptographically-outfitted mailer operated through an encrypted telnet session.
First of all, I suppose the browser has groovy bells and whistles to spice up reading email. Maybe you could run a snazzy GUI standalone mailer in some windowing protocol through the encrypted telnet link as an alternative. This is largely irrelevant to me personally, but could make a huge difference to the hoi polloi.
Ignoring such interface concerns, what kinds of situations would be conducive to the JavaMail approach but not, say, ssh+Elm+PGP ? Perhaps I'm visiting someone where there's no local POP for my ISP. The friend I'm visiting has e.g. an AOL account. I would like to read mail on my remote account via her AOL browser. Am I wrong in assuming that I couldn't execute a telnet: URL with the AOL browser ?
The difference is the ubiquity of the infrastructure. Indeed, nothing in my scenario differs on a technical basis from encrypted telnet + PGP. However, in my scenario, anybody with a Java-capable Web browser would be able to access the service, just by clicking on a link. Raph