For what it's worth, tunneling and compression services designed for generic
internet or 3G networks are not necessarily optimized for satphone (or
SSB radio) email
service.
With Satphone and
SSB, the latencies are very high, and the data rates are fairly slow, so the normal TCP/IP handshaking can really slow down the data throughput. Also, it is fairly common to drop the connection in the middle of a satphone or SSB data transfer, and typical protocols will discard the entire partially-sent message. With XGate (and no doubt other) satphone-optimized services, partially-transferred messages are saved, and at the next connection the transfer resumes where it left off. This is a hugely valuable feature.
I am aware that the "resume interrupted download" feature is available for generic
internet service, but not all transfer programs offer it. XGate provides a well-optimized service that goes far beyond simple compression.
I keep mentioning XGate because I have been using it for a number of years, and have been quite satisfied. There may be even better options out there...