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When using a mailserver I know that IMAP leaves all the mail in the server while pop3 downloads it and deletes it (sometimes after a few days, depending on the mail client configuration options). My question is, what happens when using both protocols?, for example using pop3 in my outlook client should delete the message after a few days and therefore will not be accessible anymore from webmail, but accessing the same message from webmail first leaves it there until the pop3 protocol deletes it? The fact that a message is deleted or not (and how) depends on the protocol used to read it for the first time?

Thanks in advance.

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The POP3 RFCs explicitly allow a client to leave mail on a server, but warn that it will lead to the buildup of mail, and encourage clients to send explicit delete commands after getting email, and encourage administrators to consider site-wide retention policies to delete email some time after it has been read.

In short it's not the protocol. Either your client is sending the DELE command to delete mail, maybe after a few days, or your server has a site-policy to delete mail which has been read by POP3 after some time. Neither behavior is mandated by the POP3 protocol, though.

https://tools.ietf.org/html/rfc1939 "Post Office Protocol - Version 3" includes:

  1. The UPDATE State

When the client issues the QUIT command from the TRANSACTION state, the POP3 session enters the UPDATE state. The POP3 server removes all messages marked as deleted from the maildrop [..] In no case may the server remove any messages not marked as deleted.

If a session terminates for some reason other than a client-issued QUIT command, the POP3 session does NOT enter the UPDATE state and MUST not remove any messages from the maildrop.

(emphasis mine)

and

  1. Scaling and Operational Considerations

Since some of the optional features described above were added to the POP3 protocol, experience has accumulated in using them in large-scale commercial post office operations where most of the users are unrelated to each other. In these situations and others, users and vendors of POP3 clients have discovered that the combination of using the UIDL command and not issuing the DELE command can provide a weak version of the "maildrop as semi-permanent repository" functionality normally associated with IMAP. [..]

When these facilities are used in this way by casual users, there has been a tendency for already-read messages to accumulate on the server without bound. This is clearly an undesirable behavior pattern from the standpoint of the server operator. This situation is aggravated by the fact that the limited capabilities of the POP3 do not permit efficient handling of maildrops which have hundreds or thousands of messages.

Consequently, it is recommended that operators of large-scale multi-user servers, especially ones in which the user's only access to the maildrop is via POP3, consider such options as: [..]

  • Enforce a site policy regarding mail retention on the server.

    Sites are free to establish local policy regarding the storage and retention of messages on the server, both read and unread. For example, a site might delete unread messages from the server after 60 days and delete read messages after 7 days. Such message deletions are outside the scope of the POP3 protocol and are not considered a protocol violation.

    Server operators enforcing message deletion policies should take care to make all users aware of the policies in force.

    Clients must not assume that a site policy will automate message deletions, and should continue to explicitly delete messages using the DELE command when appropriate.

and

https://tools.ietf.org/html/rfc2449

Discussion: While POP3 allows clients to leave messages on the server, RFC 1939 [POP3] warns about the problems that may arise from this, and allows servers to delete messages based on site policy.

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  • It's possible to use pop3 without deleting messages.
  • To answer your question (not sure if I got you right): What happens to the mails depends on the actions which are executed via the used protocol. It doesn't necessarily depend on the fact, which protocol was used to read the mail for the first time.

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