If you get duplicate messages, then there are several common reasons for this:
Multiple POP3 mailboxes on the ISP
If your ISP gives you one POP3 mailbox for each VPOP3 user, then you have to tell VPOP3 which user each mailbox is associated with. If you don't do this, then when a message is sent to two VPOP3 users, each user will receive two copies of that message.
To link each ISP mailbox with a VPOP3 user, go to the VPOP3 Mail Connectors → Mail Collectors page. Then, for each Mail Collector you have, in the POP3 Routing tab, choose Send all messages to a specified user/list and select the VPOP3 user who this POP3 mailbox is for.
Duplicate messages at the ISP
Sometimes the ISP will duplicate messages in their mailbox if a message is sent to multiple recipients. For instance, if a message is addressed to 'email@example.com' and 'firstname.lastname@example.org', then the ISP will put a copy of the message into their mailbox for 'bob', and another copy for 'jane'. Also, some mail sending software will do this, so if a message is sent to two people, the sending software will send the message twice.
Whatever the cause, more than one identical (or almost identical) message will be in the POP3 mailbox at the ISP. VPOP3 will download these and look at the message headers to see who the message is for. It will then send each copy of the message to each of the recipients listed in the message headers, meaning that people may get more than one copy.
The duplicate detection in VPOP3 will usually fix this problem. However, there are some times when it can not - for instance, if the sending software has assigned each copy of the message a different
message-id. In that case VPOP3 plays safe and assumes that the messages are different copies. If it didn't do so, then there would be too great a risk of messages being lost.
By default, the duplicate detection in VPOP3 is enabled, but to confirm this:
Mail collector duplicate detection. Check Mail Connectors → Mail Collectors → (name) → POP3 General → Attempt to remove duplicate messages. Note that this only looks in the current session of this mail collector, it does not look among different sessions, or different collectors
Global duplicate detection. Check Settings → Misc Settings → General → Enable Global duplicate message detection for X days
If VPOP3 gets disconnected when downloading mail from the ISP and you do NOT have Leave Messages on Server for X days enabled on the In Mail tab in VPOP3, then the next time VPOP3 connects it will download all the messages again.
This is because, although VPOP3 has asked the ISP to delete the message from the ISP POP3 mailbox the POP3 standard says that if a connection error occurs, the ISP must automatically undelete all messages which were deleted during that session.
The solution is to tell VPOP3 to Leave Messages On Server for at least one day.
Multiple Identical Mail Collectors
Check that you do not have multiple identical Mail Collectors defined in VPOP3. If you do,then remove all but one of them.
Multiple Copies of VPOP3 running
Using the Processes list in Windows Task Manager, make sure you do not have more than one copy of VPOP3.EXE running. (Click on the Show Processes for all Users button/box if there is one).
Internal messages being received multiple times
Some email clients (eg Microsoft Outlook) will silently resend messages if they don't receive a response from the mail server soon enough. This can lead to it resending a message many, many, times. This is most likely to happen with large messages and/or messages to many local recipients, because of the time taken to process these messages.
To try to alleviate these problems, some things to check are:
make sure that VPOP3 is working as fast as it can - e.g. by regularly defragmenting the VPOP3 server hard disk (unless it's an SSD).
check any virus scanner on the sender's or VPOP3 PC. If that is checking SMTP (outgoing) messages, then that can add a substantial delay to sending large messages.
increase the timeout in Outlook (in the email account's 'Advanced' settings). The default is 1 minute, even though the SMTP standards say it should be at least 10 minutes.
if the problem is when sending to a large number of local users, then consider using a Mailing List in VPOP3 instead of a distribution list or just CCing lots of people. VPOP3 processes Mailing Lists in the background so it responds to the email client much quicker.
If none of these common causes matches your problem, then you need to send a copy of a set of duplicated messages (e.g. if you receive two copies of a message, send both copies) in a support ticket to us with any other information you have (e.g. which messages are duplicated etc). We must see the FULL message headers, not just the abbreviated headers which many email clients show by default.