Invalid RCPT TO: list

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Hi

We have the following set up for mail flow;

Application (Server1) > IIS SMTP relay server (Server2) > Sendmail
SMTP relay server (Server3)

We noticed that if the application tries to send to more than one
user, the mail is not delivered.
If the application tries to send to only one user, the mail is
delivered.

We checked the application log on Server1, and noticed the following;

MAIL FROM: Application@xxxxxxxxxxxx
RCPT TO: <user1@xxxxxxxxxxxx, user2@xxxxxxxxxxxx, user3@xxxxxxxxxxxx>

This is invalid and should be

RCPT TO: <user1@xxxxxxxxxxxx>, <user2@xxxxxxxxxxxx>,
<user3@xxxxxxxxxxxx>

So far, so good....we know where the problem lies.

However, if I check the IIS log on Server2, I can see the following
entries;

EHLO - Server2 0 0 4 0 156 SMTP - - - -
2008-08-20 13:16:01 153.7.110.21 OutboundConnectionResponse
SMTPSVC1 Server2 - 25 -
- 250-Server3 0 0 19 0 313 SMTP - - -
-
2008-08-20 13:16:01 153.7.110.21 OutboundConnectionCommand SMTPSVC1
Server2 - 25
MAIL - FROM:<App@xxxxxxxxxxxx>+SIZE=688 0 0 4 0
313 SMTP - - -
-
2008-08-20 13:16:01 153.7.110.21 OutboundConnectionResponse
SMTPSVC1 Server2 - 25 -
- 250+sender+<App@xxxxxxxxxxxx>+OK 0 0 74 0 484
SMTP - - -
-
2008-08-20 13:16:01 153.7.110.21 OutboundConnectionCommand SMTPSVC1
Server2 - 25
RCPT - TO:<user1@xxxxxxxxxxxx> 0 0 4 0 484 SMTP - - -
-
2008-08-20 13:16:01 153.7.110.21 OutboundConnectionCommand SMTPSVC1
Server2 - 25
RCPT - TO:<user2@xxxxxxxxxxxx> 0 0 4 0 484 SMTP - - -
-
2008-08-20 13:16:01 153.7.110.21 OutboundConnectionCommand SMTPSVC1
Server2 - 25
RCPT - TO:<user3@xxxxxxxxxxxx> 0 0 4 0 484 SMTP - - -
-
2008-08-20 13:16:01 153.7.110.21 OutboundConnectionResponse
SMTPSVC1 Server2 - 25 -
- 250+recipient+<user1@xxxxxxxxxxxx>+OK 0 0 53 0 641 SMTP - - -
-
2008-08-20 13:16:03 153.7.110.21 OutboundConnectionResponse
SMTPSVC1 Server2 - 25 -
- 250+recipient+<user2@xxxxxxxxxxxx>+OK 0 0 53 0 1016 SMTP - - -
-
2008-08-20 13:16:03 153.7.110.21 OutboundConnectionCommand SMTPSVC1
Server2 - 25
DATA - - 0 0 4 0 1016 SMTP - - -
-
2008-08-20 13:16:03 153.7.110.21 OutboundConnectionResponse
SMTPSVC1 Server2 - 25 -
- 354+Enter+mail,+end+with+"."+on+a+line+by+itself 0 0 48 0 1172
SMTP - - - -
2008-08-20 13:16:03 153.7.110.21 OutboundConnectionResponse
SMTPSVC1 Server2 - 25 -
- 250+Message+received+and+queued 0 0 31 0 1484 SMTP - - -
-
2008-08-20 13:16:03 153.7.110.21 OutboundConnectionCommand SMTPSVC1
Server2 - 25
QUIT - - 0 0 4 0 1484 SMTP - - - -

As you can see, the recipients are broken down into seperate user and
RCPT TO: addresses.

So, my question is -

a) If the application is sending out RCPT TO: <user1@xxxxxxxxxxxx,
user2@xxxxxxxxxxxx, user3@xxxxxxxxxxxx>, why is IIS sending out (or
looks to me like it is) RCPT TO: user1@xxxxxxxxxxxx, RCPT TO:
user2@xxxxxxxxxxxx etc.

b) If IIS is sending RCPT TO: list as three users, what recipient list
does the MTA at Server3 see?

I would have thought the RCPT TO: list would stay the same as it
progressed through each MTA?

Hope someone can help!

.