IMAP and command APPEND and its parameters and RFC 2060

Can you help me to understand what is the last parameter in example of APPEND command in RFC 2060. Ther开发者_JS百科e is example C: A003 APPEND saved-messages (\Seen) {310} and what mean the {310}

Thank M.

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310 is the number of bytes in the message literal that you're appending