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RFC 3501 states:

IMAP client and server implementations MUST implement the TLS_RSA_WITH_RC4_128_MD5.

RFC 7465 states:

o TLS clients MUST NOT include RC4 cipher suites in the ClientHello message

o TLS servers MUST NOT select an RC4 cipher suite when a TLS client sends such a cipher suite in the ClientHello message.

o If the TLS client only offers RC4 cipher suites, the TLS server MUST terminate the handshake. The TLS server MAY send the insufficient_security fatal alert in this case.

I don't see anyway to reconcile these two RFCs. I have seen that Google has turned off RC4 on its email servers and is requiring TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, but that's not necessarily interoperable with the rest of the world.

Is there an update to RFC 3501 coming that changes the mandated TLS cipher suite?

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I don't see anyway to reconcile these two RFCs

There are two ways to reconcile them:

  1. Choose Security (RFC 7465) over Backwards Compatibility (RFC 3501)
  2. Choose Backwards Compatibility (RFC 3501) over Security (RFC 7465)

I have seen that Google has turned off RC4 on its email servers and is requiring TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, but that's not necessarily interoperable with the rest of the world.

Google chose option #1. They probably took a census of all their IMAP traffic, reviewed the Client Cipher Suites being suggested, saw that either nobody or a negligibly small number of clients would be impacted, and then shut off RC4.

Is there an update to RFC 3501 coming that changes the mandated TLS cipher suite?

Doubtful. RFCs require common sense. It is to be expected that narrow technical recommendations like mandated ciphers in an otherwise large specification will be overridden by later, more specific RFCs. Nobody's going to go revise the IMAP RFCs just to make sure they reflect current TLS practice; everybody is expected to vote with their feet with current TLS practice.

This is all in line with the special status of RFCs - they're rules, which everybody has to follow, except when they aren't or they don't. The only force an RFC has is the critical mass or people paying attention to it.

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RFC 7525 provides recommendations on how to deal with TLS protocols and ciphers today, including:

4.1.  General Guidelines
   ...
   o  Implementations MUST NOT negotiate RC4 cipher suites.

And it is applicable to IMAP too:

5.  Applicability Statement
   ...
   o  Email software and services that wish to protect IMAP, POP3, or
      SMTP traffic with TLS.
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