IMAP extension support

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首先查看 IMAP4rev2 清单以及现有的 MOVE 支持，该支持通过 imaptest 进行测试。在调查其实现和集成测试需求之前，先选择一个具体的、尚未勾选的扩展。完成的标准是：该扩展已被通告、客户端可以使用，并且有集成测试覆盖。

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DoD: Extension is considered to be supported if it is advertised, usable by clients and integration tests for its use exist.

Extensions that are required to be compatible with IMAP4rev2 specification.

Extensions that were removed from IMAP4rev2 but are still recommended. Widely supported and used by clients.

Extensions set from 2006 recommended for efficient operation of mobile clients ("to Support Diverse Service Environments").

Experimental extensions that bring considerable improvements.

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