imap_capabilities

Sets the IMAP protocol extensions list that is passed to the client in response to the CAPABILITY command. The authentication methods specified in the imap_auth and STARTTLS directives are automatically added to this list if the starttls directive is enabled.

It makes sense to specify the extensions supported by the IMAP backends to which the clients are proxied (if these extensions are related to commands used after the authentication, when nginx transparently proxies a client connection to the backend).

The current list of standardized extensions is published at www.iana.org.