fix long imap login delay on mailboxes with many folders

[gagarinlg]

Fixes #145

Changes proposed in this pull request:

• just check imap server capabiities instead of pulling the complete folder list

[violoncelloCH]

thank you for your pull request! would it be possible to sign-off on your commit? - see https://github.com/nextcloud/server/blob/master/.github/CONTRIBUTING.md#sign-your-work can be done with git commit --amend -s for the last commit and then git push -f to push the changed commit back to github...

What worries me a bit is that, as it is called CUSTOMREQUEST, this could be something not supported by all imap servers and therefore break setups of other users. Do you have some reference here? How is the request processed in that case? (cc @rollbrettler on this)

[gagarinlg]

The CUSTOMREQUEST is just the way to tell curl to do something different that the default request and has nothing to do with the imap protocol

[gagarinlg]

I signed the pull request.

[gagarinlg]

The "CAPABILITY" command ist mandatory for IMAP servers. see: https://tools.ietf.org/html/rfc3501, page 24: 6.1. Client Commands - Any State

The following commands are valid in any state: CAPABILITY, NOOP, and LOGOUT.

6.1.1. CAPABILITY Command

Arguments: none

Responses: REQUIRED untagged response: CAPABILITY

Result: OK - capability completed BAD - command unknown or arguments invalid

  The CAPABILITY command requests a listing of capabilities that the
  server supports.  The server MUST send a single untagged
  CAPABILITY response with "IMAP4rev1" as one of the listed
  capabilities before the (tagged) OK response.

[violoncelloCH]

thanks for the clarification and the reference @gagaringlg

[violoncelloCH]

thanks for the fix!