[sylpheed:37222] "503 AUTH command used when not advertised"

Possibly as a result of config changes at my
DNS hosting provider, my OUTBOUND email just 
died without any warning. Bundled with my
virtual web server there is a mail server 
which I don't muck with as such for lack of 
competence. 

"503 AUTH command used when not advertised"

First off, what is the meaning of this error 
message?

sylpheed.log ends with:

[14:37:25] ** LibSylph-WARNING: [14:37:25] error occurred on SMTP
session
[14:37:25] ** warning: error occurred on SMTP session
[14:37:25] ** Sylpheed-WARNING: send: error: 503 AUTH command used when
not advertised
[14:37:25] ** LibSylph-WARNING: [14:37:25] Error occurred while sending
the message.
[14:37:25] ** warning: Error occurred while sending the message.

On the Account-Settings/SSL page I used to have
all fields UNchecked, now I might be able to 
send this out only if I enable *Use STARTTLS* in 
both POP3 and SMTP boxes. I'm doing this blind but 
I think I will get a popup to accept a self-signed 
certificate from my server.

IF it does make the list I'd like to know a little 
more about it, like why should I enable SSL at all 
when *I* don't need it?

TIA