RFC 9051, "Internet Message Access Protocol (IMAP) - Version 4rev2", August 2021

Errata ID: 8001
Status: Reported
Type: Technical
Publication Format(s) : TEXT, PDF, HTML
Reported By: Yasumasa Shimizu
Date Reported: 2024-06-25

Section Appendix E says:

   1.   Support for 64-bit message and body part sizes.
...
   30.  Loosened requirements on servers when closing connections to be
        more aligned with existing practices.

It should say:

   1.  Support for 64-bit message and body part sizes.
...
   30.  Loosened requirements on servers
        when closing connections to be more aligned with existing practices.
   31.  Response of the SUBSCRIBE command and the UNSUBSCRIBE command
        is changed from tagged NO to tagged OK
        if the mailbox is already subscribed/unsubscribed.

Notes:

RFC3501 6.3.6 says:

The SUBSCRIBE command adds the specified mailbox name to the server's set of "active" or "subscribed" mailboxes as returned by the LSUB command. This command returns a tagged OK response only if the subscription is successful.

According to this, SUBSCRIBE command returns a tagged NO response if the mailbox is already subscribed.
however, RFC 9501 6.3.7 says:

The SUBSCRIBE command adds the specified mailbox name to the server's set of "active" or "subscribed" mailboxes as returned by the LIST (SUBSCRIBED) command. This command returns a tagged OK response if the subscription is successful or if the mailbox is already subscribed.

This can be said that there are not compatible between IMAP4rev1 and IMAP4rev2.
This problem also occurs in UNSUBSCRIBE command.
I think this should be written in Appendix E.

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