When looking into this @用心棒 and I have looked at the following thread on the internet.
https://dovecot.org/list/dovecot/2016-December/106382.html
IMAP Sieve is used for spam reporting purposes. Essentially Mail moved from Inbox to Spam in an IMAP client such as Outlook will mark a mail as spam and mail moved from the spam folder to the inbox will unmark it.
There are essentially two methods of adding a message to an IMAP folder (referred to as a mailbox in the IMAP documentation).
APPEND - adds a message to a folder.
From RFC 3501
The APPEND command appends the literal argument as a new message
to the end of the specified destination mailbox. This argument
SHOULD be in the format of an [RFC-2822] message. 8-bit
characters are permitted in the message. A server implementation
that is unable to preserve 8-bit data properly MUST be able to
reversibly convert 8-bit APPEND data to 7-bit using a [MIME-IMB]
content transfer encoding.
Whereas COPY - copies a mail from one folder to another.
The COPY command copies the specified message(s) to the end of the
specified destination mailbox. The flags and internal date of the
message(s) SHOULD be preserved, and the Recent flag SHOULD be set,
in the copy.
An example of APPEND would be when you send an email - the sent email is also APPENDed to your SENT folder as it does not yet exist anywhere on the server.
On the other hand mail is COPYed to the SPAM folder from the INBOX (with the INBOX copy then being deleted once the COPY is complete to give the impression the mail was moved from one folder to the other).
Some email clients such as eMClient use APPEND when moving mail from the INBOX to the SPAM folder. This is probably a case of it works so to hell with the standards, however some email servers do reject APPENDing mails to specific folders. I'm actually waiting for emClient's makers to update their program.
Tun
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