[exim] 503 AUTH command

Hi,

I've got exim 4.54 running on a Debian 3.0 machine which is now running
quite well. I can send and receive from both MS Outlook and Linux
Thunderbird, but I've installed SquirrelMail 1.4.5 and when I try to
send a message, I get this error:

Bad sequence of commands
Server replied: 503 AUTH command used when not advertised

Is this an issue with SquirrelMail, or is there something I need to
check/change in my exim configuration?

I currently have 'auth_advertise_hosts = ${if eq{$tls_cipher}{}{}{*}}'
but I've also tried changing back to 'auth_advertise_hosts = *' but that
did not resolve the error. Or am I looking at the wrong setting
altogether? Obviously I'm assuming I should be changing something in my
exim config.

Thanks in advance for your help!
Robert