My old droid-o-phone, with the default mail app and Verizon's 3G service worked fine with my somewhat-dated exim4 mail server, using TLS on port 25.
Same settings don't work on the new phone, unless I'm on WiFi. Port 25 blocked on the 4G network? OK, so make Exim also listen on port 465. Now it's not blocked, but I get the "a TLS packet with unexpected length was received" error on the server, whether using WiFi or 4G. Fiddle some more, turn WiFi back on, and discover that changing the server setting from "Use TLS" to "Use SSL" works. A test message gets through! Hooray! Now turn WiFi off, and make sure it plays nice over 4G.
Oops. All else being equal, changing from local WiFi to 4G gets me back to the "unexpected length" error.
Er... surely end-to-end encryption ought not to be affected by the network in this way... right?
Maybe I need to try K9-Mail (which I tried when the old phone was new, and there was some issue, possibly involving battery drain, but much may have changed in 4 years).