Ask SKaero said, the way you write it is wrong.
When you write it as you do, the computer interprets it as:
$rcmail_config['plugins'] = array('password'); //Okey, the $rcmail_config['plugins'] = 'password', this is cool, I'll remember that!
Then the next line comes:
$rcmail_config['plugins'] = array("pop3fetcher",); //And again, the computer thinks: Okey, the $rcmail_config['plugins'] = 'pop3fetcher', this is cool, I'll remember that!
It reads from top to bottom, so if you set a variable twice, it will only remember the last one it read!
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When writing it as SKaero said, the computer will think:
$rcmail_config['plugins'] = array('password', 'pop3fetcher'); //Okey, rcmail_config['plugins'] = BOTH password AND pop3fetcher, I'll remember both of theese!
This will atleast load your plugin, getting you one step closer, and hopefully it will load smoothly and everything start working!
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