My problem was caused by checking the Phone Email checkbox in the back office.

If this checkbox is checked then, at about line 257 of getmail.php, the if/else statement that sets the username and password fails when sending an email from an email client.

The workaround was either to uncheck the check box or to add a check for isset(eblog_phonemail).

Here's the code block that causes the problem:
[code]
if ( $Settings->get('eblog_phonemail'))
{
...
}
else
{
$userpassstring = $firstline;
$contentfirstline = '';
}

http://forums.b2evolution.net/viewtopic.php?t=9014&highlight=email+wrong+login