OK, the extended logging helped me to understand what happened ... i was trying to send mail from 1 account thru the the wrong SMTP server (ie : mail-relay !).
POP is working fine, i'm using a DefaultPopServer= for the account i use most and for the other account i use username@pop3.myisp.com.
mail retrieval works OK, don't have to be connected to the right ISP.
for sending mail, i also use a DefaultSmtpServer= for the default account, and for the other account i marked the authentication checkbox in Outlook Express and as user typed #smtp.myotherisp.com (as described in the help). of course i have to be connected to the right ISP (otherwise ; mail relay warning !)