[PLUG] why does this 553 error occur when I send mail

[Ted Kubaska (easy)]

I got to mention Joomla/VirtueMart but my mail is really about email
operation.

Joomla is a CMS and VirtueMart is an e-commerce package for it. There's
a bunch of email that goes back and forth when a customer buys stuff.

When a customer buys a product, VirtueMart sends the customer a
confirmation email. It sends this email to the email address that the
customer registered with. This works fine. 

Virtuemart then also sends an email to the store. You configure the
store's email, which in my case is a gmail account. When I look at the
mail received by that account, I see that it is from the email address
that the customer registered with. This sometimes works and sometimes
does not. 

When it does not work, the error message is 
"553 sorry, your mail was administratively denied. (#5.7.1)"
This error message is sent to the customer's email address.

It is dependent on what the customer's email address is. I have two test
customers. They are both me with different names. They are identical
except for their email addresses. For one customer (whose email address
is from a DSL provider), everything works. For the other customer (whose
email address is from a cable provider), it fails with that 553 error. I
have both DSL and cable providers.

If I use an email client (Evolution) I can mail from the customer's
cable address (which is my own cable email address) to the store's email
address (which is a gmail address).

I've talked to both my hoster (godaddy) and my ISP (comcast) and neither
is able to help. And I've posted on the VirtueMart forum with more
questions about the internals of VirtueMart in that post. But I don't
think it's a VirtueMart problem ... I think it's something that comcast
is doing.

And so I'm looking at ways to modify the PHP code that does the sending
(PHPmailer is included with VirtueMart) to workaround the problem, but
this is difficult because I don't know why I see this problem and hence
where I should look.
 -ted