[imp] IMAP Connection Issue: "no auth attempts"

[Alexander Lasley]

After installing IMP, testing the IMAP connection via /test.php?app=imp 
is successful; "Namespace Information" and "IMAP server capabilities" 
are shown to me. However, connections through the web interface fail. I 
have tried two different things:

1) Logging in to the mail account while logged in to Horde as the 
default "Administrator" user. This fails and produces the message "User 
Administrator is not authorized for Mail" in the logs. I stopped here 
because this is ultimately not the way I intend to use Horde/IMP anyway.
2) Using IMP as the application to handle authentication for Horde. This 
fails and produces slightly more useful information in the logs.

Web server information:
CentOS 6.6
nginx 1.0.15 installed via yum *
php-fpm 5.4.37 installed via yum
Horde 5.2.3 installed via PEAR
IMP 6.2.7 installed via PEAR

Mail server information:
CentOS 6.6
dovecot 2.0.9 installed via yum

* I know that nginx isn't officially supported, but my issue does not 
seem related to the web server; no relevant error messages are showing 
up in nginx's log.

I've replaced sensitive information in the logs with the following:
Horde installed into the document root /path/to/horde
Horde running on webmail.domain.tld with IP address xxx.xxx.xxx.xxx
Dovecot/Postfix running on mail.domain.tld with IP address yyy.yyy.yyy.yyy
Attempting to log in as username at mail.domain.tld
My personal IP zzz.zzz.zzz.zzz

Dovecot logs:
Feb 11 06:40:46 mail dovecot: auth: Debug: Loading modules from 
directory: /usr/lib64/dovecot/auth
...
Feb 11 06:40:46 mail dovecot: auth: Debug: auth client connected (pid=20366)
Feb 11 06:41:16 mail dovecot: imap-login: Disconnected (no auth 
attempts): rip=xxx.xxx.xxx.xxx, lip=yyy.yyy.yyy.yyy, TLS handshaking: 
Disconnected

Horde logs:
Feb 11 06:39:36 web-ln1 HORDE: [imp] [login] Error when communicating 
with the mail server. [pid 3243 on line 730 of 
"/path/to/horde/imp/lib/Imap.php"]
Feb 11 06:39:36 web-ln1 HORDE: [imp] FAILED LOGIN for 
username at mail.domain.tld (zzz.zzz.zzz.zzz) to 
{imap://mail.domain.tld:993/} [pid 3243 on line 157 of 
"/path/to/horde/imp/lib/Auth.php"]
Feb 11 06:39:36 web-ln1 HORDE: [horde] FAILED LOGIN for 
username at mail.domain.tld to horde (24.107.154.151) [pid 3243 on line 199 
of "/path/to/horde/login.php"]

/path/to/horde/imp/config/backends.php:
$servers['imap'] = array(
     'disabled' => false,
     'name' => 'IMAP Server',
     'hostspec' => 'mail.domain.tld',
     'hordeauth' => false,
     'protocol' => 'imap',
     'port' => 993,
     'secure' => 'tls',
);

Dovecot is configured to expect 'PLAIN' or 'LOGIN' authentication over 
TLS. I can't seem to find any documentation on setting the IMAP 
authentication mechanism. But IMP doesn't seem to be passing along 
incorrect credentials; it doesn't seem to be passing any credentials at all.

Thanks for your time,

Alex