[Evolution] Help with "Error while sending message"

[Formyx Amplifiers <info formyxamps com>]

Hi,

I have been using Evolution for about a year now and some months ago
while trying to send an email with images attached to a Gmail account,
I got this Warning:

"Error while Sending message.

RCPT TO <xxxxxxxx gmail com> failed: Please turn on SMTP Authentication
in your mail client. 
h83-209-157-75.cust.xx.alltele.xx ([192.168.2.51])
[xx.xxx.xx.xxx]:36718
is not permitted to relay through this server without authentication."


The warning has been there ever since I Send/Receive messages. I tried
to find the relevant message in my Outbox in order to delete it but it
simply is not there, or in any other folder.
Tried the manual and the Net but there was no relevant reference to
Evolution.

Does anyone have a better clue as to what I have to do to get rid of
this warning OR how to find the blocked email?


All Best,
Klearchos

Formyx Amplifiers