imap_auth plain;

Sets permitted methods of authentication for IMAP clients. Supported methods are:

login

AUTH=LOGIN

plain

AUTH=PLAIN

cram-md5

AUTH=CRAM-MD5. In order for this method to work, the password must be stored unencrypted.

imap_capabilities IMAP4 IMAP4rev1 UIDPLUS;

Allows to specify the IMAP protocol extensions list to be passed to the client upon issuing the CAPABILITY command. 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 extensions supported by IMAP backends to which clients are proxied (if this extensions are related to commands used after the authentication, when nginx transparently proxies the client connection to the backend).

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

imap_client_buffer 4k|8k;

Sets the IMAP commands read buffer size. By default, the buffer size is equal to one memory page. This is either 4K or 8K, depending on a platform.