I agree fully except:

Julian Mehnle wrote:

Think about how difficult it is to get that right. Sure, the client (MUA) could sent to the others and after the user corrected the address send it to the address that was originally faulty. It could even include the full cc list in that message. However, it has no way to get the cc list correct in the first messages sent, one address is not correct. Think about the suprise the users face when the same mail has different cc-lists when delivered to different addresses. No, not sending the message at all in the first place is correct (as in prinicple of least surprise).

I do fully agree that courier should reject the recipient outright. In fact, one client now relays through my courier so these addresses are rejected outright instead of generating a DSN somewhere later along the line (has something to do with the mailsetup from my otherwise excelent ISP). Educating the users is a pain, but as long as there are broken MUAs one always has to decide to educate or mechanically fix. This is a case where education pays in the end, as there is no end to the problems these broken MUAs create imho.

<rant> Another case of this is outlook that lets users copy email addresses with the '[]' around them. These addresses are wrong underneath -- just open the properties pane -- but are visually identical to correct addresses otherwise. Another reason why I hate outlook, one I have to deal with on a regular basis. </rant>

Cheers, M4