POP

Post Office Protocol

Connect, pickup, disconnect and read messages offline.
Messages can be deleted as they are picked up or held on the server
Originally used port 110 but with increased intrest in security, now can use TLS to port 995.
Example using openssl:

openssl s_client -connect pop.verizon.net:995 
CONNECTED(00000006)

depth=2 C = US, O = DigiCert Inc, OU = www.digicert.com, CN = DigiCert High Assurance EV Root CA
verify return:1
depth=1 C = US, O = DigiCert Inc, OU = www.digicert.com, CN = DigiCert SHA2 High Assurance Server CA
verify return:1
depth=0 C = US, ST = California, L = Sunnyvale, O = Oath Inc, CN = pop.aol.com
verify return:1
---
Certificate chain
 0 s:/C=US/ST=California/L=Sunnyvale/O=Oath Inc/CN=pop.aol.com
   i:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 High Assurance Server CA
 1 s:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 High Assurance Server CA
   i:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert High Assurance EV Root CA
---
Server certificate
-----BEGIN CERTIFICATE-----
MIIHJzCCBg+gAwIBAgIQAtaLNch9n+8uMlTTprqrjjANBgkqhkiG9w0BAQsFADBw
…
CY4rC/G35FWE0/T5JQCyNJwIaQSJqprjCWVNPs6W1PKM7EAMwNbGtNwdgmGFt2Bk
BnduvwD5AADtAC8=
-----END CERTIFICATE-----
subject=/C=US/ST=California/L=Sunnyvale/O=Oath Inc/CN=pop.aol.com
issuer=/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 High Assurance Server CA
---
No client certificate CA names sent
Server Temp Key: ECDH, X25519, 253 bits
---
SSL handshake has read 3513 bytes and written 289 bytes
---
New, TLSv1/SSLv3, Cipher is ECDHE-RSA-AES256-GCM-SHA384
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : ECDHE-RSA-AES256-GCM-SHA384
    Session-ID: E3B0C44298FC1C149AFBF4C8996FB9242…
    Session-ID-ctx: 
    Master-Key: 87598B2FA6A32F0E4FE676367C0EF311E…
    TLS session ticket lifetime hint: 60 (seconds)
    Start Time: 1641991269
    Timeout   : 7200 (sec)
    Verify return code: 0 (ok)

---
S:  +OK Hello from jpop-0.1
C:  user magdaman@domain.org
s:  +OK Password required.
C:  pass myEmailPassword
S:  +OK magdaman's maildrop has 2 messages (320 octets)
C:    STAT
S:    +OK 2 320
C:    LIST
S:    +OK 2 messages (320 octets)
S:    1 120
S:    2 200
S:    .
C:    RETR 1
S:    +OK 120 octets
S:    
S:    .
C:    DELE 1
S:    +OK message 1 deleted
C:    RETR 2
S:    +OK 200 octets
S:    
S:    .
C:    DELE 2
S:    +OK message 2 deleted
C:    QUIT
S:    +OK dewey POP3 server signing off (maildrop empty)
C: 
S: