My web host have screwed something up and now anyone can send email from my domain whether the sending email address is valid or not and whether the password is valid or not.

Please help me to make them understand what they have done (or undone) to cause this!

how do you know anybody can send through your domain.

Anybody that sends an email can make it look like it came from wherevever they want.

Spammers use this alot, I even get mail that looks like its come from me. All the spammers do is change the from details in thier email options. The chances are they are not actually sending it from your domain

Sending emails is not restricted to your ISP, if I know your domain, I too can set my Outlook to say it is sending from your domain while I am really sending from my own SMTP server via my own ISP services.

Are you sure the email is going out from your own website? Do you have a Webmail service on your site ? Have you tried login in to it yourself with a wrong user name and password ? If you can, simply document your steps in an email and mail it to your support explaining your problem and how to replicate it,

If you can not replicate the problem, I would think it is spammers who has recorded your email address and are flooding others under your name. I get that done to me as well. I simply ask the people to look at the Email headers to see where it really came from (SMTP server).