I am trying to use dovecot without ssl in centos 7. It works with imap but I am not able to make it work with pop3. Pop3 only works with ssl=yes. I can use dovecot in centos 4 ok.  I get the same error with thunderbird and outlook. But for some reason it works when I telnet to port 110 and do retr 1. I was able to work pam and shadow passwords.

this is the output that does not work with tls disabled

Oct 18 13:30:17 auth: Debug: Loading modules from directory: /usr/lib64/dovecot/auth
Oct 18 13:30:17 auth: Debug: Module loaded: /usr/lib64/dovecot/auth/lib20_auth_var_expand_crypt.so
Oct 18 13:30:17 auth: Debug: Module loaded: /usr/lib64/dovecot/auth/libdriver_sqlite.so
Oct 18 13:30:17 auth: Debug: Read auth token secret from /var/run/dovecot/auth-token-secret.dat
Oct 18 13:30:17 auth: Debug: auth client connected (pid=22513)
Oct 18 13:30:17 auth: Debug: client in: AUTH    1       PLAIN   service=pop3    session=WXv1I4R4FOXAqAAK        lip=192.168.0.37        rip=192.168.0.10    lport=110        rport=58644
Oct 18 13:30:17 auth: Debug: client passdb out: CONT    1
Oct 18 13:30:17 auth: Debug: client in: CONT    1       == (previous base64 data may contain sensitive data)
Oct 18 13:30:17 auth-worker(22515): Debug: Loading modules from directory: /usr/lib64/dovecot/auth
Oct 18 13:30:17 auth-worker(22515): Debug: Module loaded: /usr/lib64/dovecot/auth/lib20_auth_var_expand_crypt.so
Oct 18 13:30:17 auth-worker(22515): Debug: Module loaded: /usr/lib64/dovecot/auth/libdriver_sqlite.so
Oct 18 13:30:17 auth-worker(22515): Debug: shadow(joe2,192.168.0.10,<WXv1I4R4FOXAqAAK>): lookup
Oct 18 13:30:17 auth: Debug: client passdb out: OK      1       user=joe2
Oct 18 13:30:17 auth: Debug: master in: REQUEST 2718302209      22513   1       05eac7a2917ce3bc1f0f359b21b138e6        session_pid=22516
Oct 18 13:30:17 auth-worker(22515): Debug: passwd(joe2,192.168.0.10,<WXv1I4R4FOXAqAAK>): lookup
Oct 18 13:30:17 auth: Debug: master userdb out: USER    2718302209      joe2    system_groups_user=joe2 uid=660 gid=661 home=/home/joe2
Oct 18 13:30:17 pop3(joe2)<22516><WXv1I4R4FOXAqAAK>: Debug: Effective uid=660, gid=661, home=/home/joe2
Oct 18 13:30:17 pop3(joe2)<22516><WXv1I4R4FOXAqAAK>: Debug: Namespace inbox: type=private, prefix=, sep=, inbox=yes, hidden=no, list=yes, subscriptions=yes location=mbox:~/mail/:INBOX=/var/mail/joe2
Oct 18 13:30:17 pop3(joe2)<22516><WXv1I4R4FOXAqAAK>: Debug: fs: root=/home/joe2/mail, index=, indexpvt=, control=, inbox=/var/mail/joe2, alt=
Oct 18 13:30:17 pop3(joe2)<22516><WXv1I4R4FOXAqAAK>: Debug: Mailbox INBOX: Mailbox opened because: POP3 INBOX
Oct 18 13:30:17 pop3(joe2)<22516><WXv1I4R4FOXAqAAK>: Debug: Mailbox INBOX: UID 30: Opened mail because: access
Oct 18 13:30:17 pop3(joe2)<22516><WXv1I4R4FOXAqAAK>: Debug: Mailbox INBOX: UID 30: Opened mail because: RETR


This is the output with TLS and it works

Oct 18 13:30:32 auth: Debug: auth client connected (pid=22542)
Oct 18 13:30:33 auth: Debug: client in: AUTH    1       PLAIN   service=pop3    secured=tls     session=44feJIR4FeXAqAAK        lip=192.168.0.37        rip=192.168.0.10     lport=110       rport=58645     ssl_cipher=ECDHE-RSA-AES128-GCM-SHA256  ssl_cipher_bits=128     ssl_pfs=ECDH    ssl_protocol=TLSv1.2
Oct 18 13:30:33 auth: Debug: client passdb out: CONT    1
Oct 18 13:30:33 auth: Debug: client in: CONT    1       == (previous base64 data may contain sensitive data)
Oct 18 13:30:33 auth-worker(22515): Debug: shadow(joe2,192.168.0.10,<44feJIR4FeXAqAAK>): lookup
Oct 18 13:30:33 auth: Debug: client passdb out: OK      1       user=joe2
Oct 18 13:30:33 auth: Debug: master in: REQUEST 4165992449      22542   1       cb03b70f92c0c5c030e25a39ea4b8518        session_pid=22543
Oct 18 13:30:33 auth-worker(22515): Debug: passwd(joe2,192.168.0.10,<44feJIR4FeXAqAAK>): lookup
Oct 18 13:30:33 auth: Debug: master userdb out: USER    4165992449      joe2    system_groups_user=joe2 uid=660 gid=661 home=/home/joe2
Oct 18 13:30:33 pop3(joe2)<22543><44feJIR4FeXAqAAK>: Debug: Effective uid=660, gid=661, home=/home/joe2
Oct 18 13:30:33 pop3(joe2)<22543><44feJIR4FeXAqAAK>: Debug: Namespace inbox: type=private, prefix=, sep=, inbox=yes, hidden=no, list=yes, subscriptions=yes location=mbox:~/mail/:INBOX=/var/mail/joe2
Oct 18 13:30:33 pop3(joe2)<22543><44feJIR4FeXAqAAK>: Debug: fs: root=/home/joe2/mail, index=, indexpvt=, control=, inbox=/var/mail/joe2, alt=
Oct 18 13:30:33 pop3(joe2)<22543><44feJIR4FeXAqAAK>: Debug: Mailbox INBOX: Mailbox opened because: POP3 INBOX
Oct 18 13:30:33 pop3(joe2)<22543><44feJIR4FeXAqAAK>: Debug: Mailbox INBOX: UID 30: Opened mail because: access
Oct 18 13:30:33 pop3(joe2)<22543><44feJIR4FeXAqAAK>: Debug: Mailbox INBOX: UID 30: Opened mail because: RETR
Oct 18 13:30:33 pop3(joe2)<22543><44feJIR4FeXAqAAK>: Debug: Mailbox INBOX: UID 31: Opened mail because: access
Oct 18 13:30:33 pop3(joe2)<22543><44feJIR4FeXAqAAK>: Debug: Mailbox INBOX: UID 31: Opened mail because: RETR