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<p>As mentioned earlier, it is possible for a client to request
attributes or contents of messages that are no longer
available. Another application might have removed the message
from the folder, and the SMAP client has not polled the server
for changes to the folder's contents.</p>
<p>With one exception, the server should ignore requests for
attributes or content of messages that are no longer available.
When the client requests attributes or content of multiple
messages, the unavailable messages should simply be omitted
from the server's single or multi-line replies. When the client
requests a single message, which is no longer available, the
server may reply with either an explicit error message, or just
a good status line reply, but without any single or multi-line
replies containing the requested data.</p>
<p>Another acceptable response in this situation is for the
server to return default data. The server can synthesize a
short, generic message whose sole contents is the text "This
message is no longer available.", and respond to queries as if
this message has been put in place of the requested
message.</p>
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<h3 class="title">Note</h3>
<p>Here's the exception. The server should always remember
the unique identifiers of messages. The server should always
respond to requests for <code class="literal">UID</code>s,
even if the message is no longer available. No two messages
in a folder will ever have the same <code class="literal">UID</code>, so memorizing them should not be too
difficult.</p>
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