How to fix Lion 10.7 Apple Webmail "Connection to IMAP server failed"? Apple webmail on Lion (10.7) Server is returning a popup error message about 60 seconds after trying to login:


  Connection to imap server failed


or in Dutch:


  Connectie met IMAP server mislukt


Regression:


Using the same user name and password, while sending and receiving mail works from Mail.app.


This Lion Server:


runs 10.7.5
is located behind a NAT router device
id configured on a non routable 192.168.0.1 IP address
uses a "Computer Name" and "Host Name" that are identical
has a "Computer Name" and "Host Name" that are in the form of "host.domain.tld"
"host.domain.tld" has an A record in public DNS that refers to the public IP address of the NAT router 


How to fix this 'Connection to IMAP server failed' issue?
The Apple Webmail "Connection to IMAP server failed" error kept re-appearing, I changed /usr/share/webmail/config/appleoverrides.inc.php to:

$rcmail_config['default_host'] = 'localhost';
//$rcmail_config['smtp_server'] = '%h';


Idea from

Change in /usr/share/webmail/config/appleoverrides.inc.php the value of rcmail_config['default_host'] from its default 'tls://%h' to 'tls://macserver.examle.com'.

source: https://discussions.apple.com/message/16922962