Can not send emails - receiving ok

My em email was working perfectly - until Monday - can no longer send emails. Keep getting the error:
ERROR MESSAGE SAYS SEE "http://pobox/~djb/docs/smtplf.html
I tried uninstalling / repairing / reinstalling…nothing works. I can receive emails but can not send out emails. What’s wrong and how to get Em Client email to work? If not, then will have to get another email softare?

What is the error, please?

I can not send emails…
the error log shows:
"Cannot send message (Email body not accepted due to the following reason:
“See http://pobox.com/~djb/docs/smtplf.html

Thanks. The first link didn’t work. Don’t know why cause they look the same.

It looks like your antivirus application is inserting unsupported text into the message.

Completely disable AVG/Avast and try send another message.

HI Gary ` THANK YOU!! I UNINSTALLED THE AVAST ANTI-VIRUS AS YOU YOU HAVE NOTED AND IT IS NOW WORKING PERFECT AS BEORE - THANK YOU!!!

You are awecome!!!