The entire email transation between a sender and a recipient usually looks like this:
Sending Client [--A---> Sender SMTP Server [--B---> Recipient SMTP Server [--C--> Recipient IMAP/POP server <---D----] Recipient Client
Connections A and D are easily possible to encrypt, provided your provider provides SSL/TLS on their SMTP and IMAP/POP servers. Most usually do. Connection C is usually local to a single machine, or for large email providers will go over an intranet of some kind.
What is at issue is connection B, which goes over the public internet. That is almost always in clear text, as most of this infrastructure was designed 30 years ago and hasn't evolved much since then. If you are sending email within a single provider (e.g. [email protected] to [email protected]), such delivery can be trivially encrypted.
Sending Client [--A---> Sender SMTP Server [--B---> Recipient SMTP Server [--C--> Recipient IMAP/POP server <---D----] Recipient Client
Connections A and D are easily possible to encrypt, provided your provider provides SSL/TLS on their SMTP and IMAP/POP servers. Most usually do. Connection C is usually local to a single machine, or for large email providers will go over an intranet of some kind.
What is at issue is connection B, which goes over the public internet. That is almost always in clear text, as most of this infrastructure was designed 30 years ago and hasn't evolved much since then. If you are sending email within a single provider (e.g. [email protected] to [email protected]), such delivery can be trivially encrypted.