Receive duplicate emails and error messages: pop

depends on a lot of things.
what defines message - an email ?
a chat program ?

speed of each hop through the internet
how many hops are needed to get to destination pop or imap server ?
speed of the mail server how many other people are sending messages through the SMTP sending email server to send else where .?
Speed of the POP destination server and how many people are receiving emails at the same time ?
Amount of internet traffic going through each hop at the time you send your message.
Depends on if a fishing trawler has ripped up the internet cable under the sea with its anchor at the time you are sending ?
Depends on also if a hacker has created a derail of service attack on the email server at the time you are sending your email ?

Depends on if receiving computer is polling the pop or imap server as well looking for new messages.

Could take 30 seconds to 30 minutes or 2 days if a fault has occurred.

hi
Most common reason for 'Outlook cannot connect to SMTP server' error is the wrong configuration settings of Outlook. This happens especially in the case of newly configured email accounts.
A mis-spelled mail server name to a wrong port setting in Outlook can cause connectivity issues to the mail server and end up giving error while trying to send or receive mails.
By default, the port settings for mail related services are these, and a connection problem to these ports can lead to Outlook error:
SMTP - port 25 / 587 POP3 - port 110 IMAP - port 143 Secure SMTP (SSMTP) - port 465 IMAP4 over SSL (IMAPS) - port 993 Secure POP3 (SSL-POP) - port 995 There are also chances that some web masters may have duplicate profiles in Outlook, which will have an old mail server configured in it, and therefore would be trying to connect to the wrong mail server.

When you use Microsoft Outlook Express to send and receive mail to a Post Office Protocol 3 (POP3) or Internet Message Access Protocol (IMAP) server running Exchange 2000 Server or Exchange Server 2003, you may receive the following error message. Your server has unexpectedly terminated the connection. Possible causes for this include server problems, network problems, or a long period of inactivity.

The problem is clearly indicated on the error code.

"the server responded 521 user is over the limit for messages to be sent in a single day."

Have you been sending a lot of messages using that account? From yahoo:

The limit for messages for yahoo is 100 per hour, max 500 per day.

If you believe that you are the one not doing this, you can either report this to yahoo or checked your Yahoo! account immediately. Change the security settings and most specially the passwords. It could be that your account is used as a spam bot.

Go here and follow instructions

http://email.about.com/od/outlooktips/qt/et_getyahoofree.htm

please go through this link ...this will solve ur issue....
http://help.yahoo.com/l/uk/yahoo/mail/yahoomail/pop/pop-08.html

Error 721: Cannot connect to the Pop Server. (Incorrect POP Server IP Address) Pop Server is down.

Solution: Check the Pop Server IP Address. Contact the Network Administrator

Were you setting up email by chance, you need to make sure the email server settings are set correctly for your POP email provider. Otherwise you will get this message. If you don't use email, go into settings (Fax Parameters) and disable email features.

Tools > Account settings > E mail accounts > Change > More settings > Outgoing server > Check the box which says > My outgoing server says authentication . Finish