[Mimedefang] Re: Great open letter about anti-virus, emails, bounces, etc.

[Tony Langdon]

> We've had good luck with kicking back a 550 that simply says 
> "Mailing directly
> from dynamics refused; please use your provider's outgoing 
> mail server to
> relay."  Even if they're a fly-by-night keeping costs down, 

The poroblem with this is you get sites like mine which send mail
(legitimately) from a couple of domains.  Using the ISP's mail server isn't
always an option, because they may refuse to send mail from the alternate
domain, and even if they did accept it, then that may still cause problems
if you attempt to look up the PTR or MX records for the source domain and
see that there's still a mismatch.

There are two legitimate reasons why an end user may be sending mail
directly to other hosts from their system.

1.  They have accounts in multiple domains, which they access through a
single connection.
2.  They are using a local mail proxy across a LAN, so their MUA isn't tied
up for extended periods when they send large attachments.

Of course, in the single domain scenario, they can define a smart relay host
(i.e. their ISP) to cover the second case and have their local mail relay
use the ISP's mail servers.

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