[Mimedefang] SMTP Header Syntax question

[Cormack, Ken]

Group,

I'm googling through the RFCs trying to find this (mainly RFC821 and 822),
but perhaps one of you might be able to help narrow this down?  It's not a
MIMEDefang question, per se, but we've got a customer trying to send us a
programmatically-generated email, that is being blocked by our gateways.
The arg being passed (to a sendmail rule that we have) indicates that the
HEADER "To:" line has incorrect syntax as follows:

	user1 at domain,user2 at domain,user3 at domain

Note the presence of commas, but and the absence of spaces, in the above.

I'm guessing that spaces are needed, but would like to be able to reference
an RFC syntax quotation for the external developer, since both they and we
maintain nothing has changed.

The particular sendmail rule we use is even older than our MIMEDefang
deployment here, (has been in service and untouched for approx 6 years now).
And I have a feeling the customer's program code has changed.  Without
calling him a liar, I'd rather just be able to say "RFC such-and-such,
section blah-blah-blah says this..."

Can anyone point me to a good reference on the structure of SMTP headers,
specifically the Header "To:" field, and the allowed syntax for multiple
recipients?

Thanks!

Ken