8. Вопросы эксплуатации и масштабируемости
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» unctionality normally associated with IMAP. Of course the other capabilities of IMAP, such as polling an existing connection for newly arrived messages and supporting multiple folders on the server, are not present in POP3.
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:
Imposing a per-user maildrop storage quota or the like.
A disadvantage to this option is that accumulation of messages may result in the user's inability to receive new ones into the maildrop. Sites which choose this option should be sure to inform users of impending or current exhaustion of quota, perhaps by inserting an appropriate message into the user's maildrop.
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.
It should be noted that enforcing site message deletion policies may be confusing to the user community, since their POP3 client may contain configuration options to leave mail on the server which will not in fact be supported by the server.
One special case of a site policy is that messages may only be downloaded once from the server, and are deleted after this has been accomplished. This could be implemented in POP3 server software by the following mechanism: «following a POP3 login by a client which was ended by a QUIT, delete all messages downloaded during the session with the RETR command». It is important not to delete messages in the event of abnormal connection termination (ie, if no QUIT was received from the client) because the client may not have successfully received or stored the messages. Servers implementing a download-and-delete policy may also wish to disable or limit the optional TOP command, since it could be used as an alternate mechanism to download entire messages.