The POP3 program which is used on CentOS is called    Dovecot. The default port for POP3 is  110.
So the first thing to check is whether you can talk to port 110

telnet   localhost  110

[root@server ~]# telnet localhost 110
Trying 127.0.0.1...
Connected to localhost.localdomain (127.0.0.1).
Escape character is '^]'.
+OK dovecot ready.
quit
+OK Logging out

A common problem with users not receiving mail is their mailbox becomes very large. Its important to remember that the POP3 protocol is not meant to handle huge mailboxes. Imagine trying to send a 100KG parcel via the postal service, the poor postman may struggle a little, but he will cope fine with 50 small envelopes.

If you need to send huge files, there are other means to do so.

The way that POP3 works, is as follows. When a user clicks receive, the mail client will authenticate, then the server will take the users mailbox, and make a copy of it, then once the copy is ready, the user will start receiving mail from the  COPY of his mailbox. This is where things normally go awry.
If a users mailbox is very big, it will take a while for the mailbox to be copied, if there is any interuption during this process, it will get stuck before its finished, and you will then be stuck with a POP lock.  Some of the users mail will be in the  half copied mailbox, and the rest in the normal mailbox. If the user trys to check mail again, he will receive an error which says he has a corrupted mail drop.


-rw-rw----        1 john         mail       1547253658 Dec 20 07:46     john
-rw-rw----        1 john         mail       255468 Dec 20 07:46     .john.pop
-rw-rw----        1 henry       mail       0 Dec 20 07:52 henry
-rw-rw-rw-     1 james       mail       0 Dec 20 07:46 james
-rw-rw----       1 sam           mail       0 Dec 19 09:24 sam


Sometimes, if your just leave it for a while, the pop lock will clear itself, if it doesn't, you will have to manually manipulate the persons mailbox to clear it. To do this, is just a matter of renaming the pop file, then getting the user to try again.

[root@server mail]# cd /var/spool/mail
[root@server mail]# mv .john.pop  john.old

Now the pop file will not stop the user from collecting the mail. Once he has received the mail, remember that you still have some of his mail sitting in the  john.old file, which you need to copy back.

[root@server mail]# cat   john.old >>  john

The above command will append the contents of the  john.old file to  johns mailbox.