Tagged commands and selected mailboxes

Every IMAP client command begins with a tag chosen by the client, such as A001. The final server response repeats that tag so several outstanding commands can be matched.

Untagged responses begin with * and carry data or state changes. A continuation request begins with +.

After authentication, LIST discovers mailboxes and SELECT INBOX opens one for read-write access. EXAMINE opens a mailbox read-only.

Tags make concurrency readable:

C: A001 LIST "" "*"
S: * LIST (\HasNoChildren) "/" "INBOX"
S: A001 OK LIST completed
C: A002 SELECT INBOX
S: * 42 EXISTS
S: * OK [UIDVALIDITY 93842] UIDs valid
S: A002 OK [READ-WRITE] SELECT completed

The untagged lines are data. A001 OK and A002 OK complete their matching commands. NO means a valid command could not be performed; BAD means a protocol or syntax problem.

Mailbox names and hierarchy delimiters come from the server. Do not split every mailbox name on /; the LIST response states the delimiter, and a NIL delimiter means no hierarchy is available.

SELECT returns synchronization evidence such as message count, UIDVALIDITY, and often UIDNEXT. Cache those values with the mailbox. A successful selection of the same display name does not prove its UID generation stayed the same.

Add a second command tag before A001 completes. Then match every final response to its command and separate untagged data from completion status.