We have just switched ISP, our new ISP uses Zimbra Server and we use VPOP to manage our mails internally. WE are able to send mails out using POP3 but we can not accept mail from the Zimbra Server using SMTP.

VPOP reports an error that VPOP3 has encountered an error whilst retrieving messages from your ISP
POP3 server (mtll (1))
The error received from the ISP was: "-ERR invalid username/password"
The last command sent by VPOP3 was: "PASS *******

This problem is usually an ISP problem which VPOP3 is just forwarding on to you.
Check with your ISP for more information.

Please help our mails are stuck with the ISP