My application sends email using the IP address and credentials setup by our customer. For one customer who uses a Sendmail on a fedora server, the Smtp Authentication is never sent. In most cases the Authentication module is called by the SmtpClient and automatically sends the authentication. For this one SMTP server that authentication never happens and I get an exception in Send() - Mailbox unavailable. The server response was: 5.7.1 <abc@def.com>... Relaying denied.

Why did the Authentication not happen?

Here's the code used to send the message... the r_xxxx variables are passed in.
        Dim scSMTP As New SmtpClient()
        With scSMTP
            .Host = r_sHost
            '
            ' Special SSL check for gmail 
            '
            If (.Host.ToLower.Contains("gmail.com") AndAlso r_iPort = 0) Then
                .EnableSsl = True
                .Port = 587 ' // or 465
            Else
                If (r_iPort <> 0) Then
                    .Port = r_iPort
                End If
                If (r_bSSL) Then
                    .EnableSsl = True
                End If
            End If

            If (Not String.IsNullOrEmpty(r_sUserName)) Then
                Dim smtpUser As New System.Net.NetworkCredential(r_sUserName, r_sPassword)
                .UseDefaultCredentials = False
                .Credentials = smtpUser
                .DeliveryMethod = SmtpDeliveryMethod.Network
            End If
            .Send(scMailMsg)
        End With
        scSMTP = Nothing



Here is a log of the attempted send. Notice how it goes from EHLO to SEND without the normal authentication negotiation (which I see for other SMTP servers):
System.Net Verbose: 0 : [5576] SmtpClient::.ctor()
System.Net Information: 0 : [5576] Associating SmtpClient#56956049 with SmtpTransport#44848203
System.Net Verbose: 0 : [5576] Exiting SmtpClient::.ctor() 	-> SmtpClient#56956049
System.Net Verbose: 0 : [5576] SmtpClient#56956049::Send(MailMessage#23415467)
System.Net Information: 0 : [5576] SmtpClient#56956049::Send(DeliveryMethod=Network)
System.Net Information: 0 : [5576] Associating SmtpClient#56956049 with MailMessage#23415467
System.Net Information: 0 : [5576] Associating SmtpTransport#44848203 with SmtpConnection#15363431
System.Net Information: 0 : [5576] Associating SmtpConnection#15363431 with ServicePoint#18760158
System.Net.Sockets Verbose: 0 : [5576] Socket#27936619::Socket(InterNetwork#2)
System.Net.Sockets Verbose: 0 : [5576] Exiting Socket#27936619::Socket() 
System.Net.Sockets Verbose: 0 : [5576] Socket#27936619::Connect(189:25#-1123664952)
System.Net.Sockets Verbose: 0 : [5576] Exiting Socket#27936619::Connect() 
System.Net Information: 0 : [5576] Associating SmtpConnection#15363431 with SmtpPooledStream#46749233
System.Net.Sockets Verbose: 0 : [5576] Socket#27936619::Receive()
System.Net.Sockets Verbose: 0 : [5576] Data from Socket#27936619::Receive
System.Net.Sockets Verbose: 0 : [5576] 00000000 : 32 32 30 20 64 6F 6E 6A-61 63 6F 62 73 61 75 74 : 220 xxxxxxxxxxxx
System.Net.Sockets Verbose: 0 : [5576] 00000010 : 6F 2E 6E 65 74 20 45 53-4D 54 50 20 53 65 6E 64 : xxxxx ESMTP Send
System.Net.Sockets Verbose: 0 : [5576] 00000020 : 6D 61 69 6C 20 38 2E 31-33 2E 31 2F 38 2E 31 33 : mail 8.13.1/8.13
System.Net.Sockets Verbose: 0 : [5576] 00000030 : 2E 31 3B 20 57 65 64 2C-20 32 20 53 65 70 20 32 : .1; Wed, 2 Sep 2
System.Net.Sockets Verbose: 0 : [5576] 00000040 : 30 30 39 20 31 30 3A 32-35 3A 34 32 20 2D 30 34 : 009 10:25:42 -04
System.Net.Sockets Verbose: 0 : [5576] 00000050 : 30 30 0D 0A                                     : 00..
System.Net.Sockets Verbose: 0 : [5576] Exiting Socket#27936619::Receive() 	-> 84#84
System.Net.Sockets Verbose: 0 : [5576] Socket#27936619::Send()
System.Net.Sockets Verbose: 0 : [5576] Data from Socket#27936619::Send
System.Net.Sockets Verbose: 0 : [5576] 00000000 : 45 48 4C 4F 20 31 36 37-31 37 33 2D 31 0D 0A    : EHLO 167173-1..
System.Net.Sockets Verbose: 0 : [5576] Exiting Socket#27936619::Send() 	-> 15#15
System.Net.Sockets Verbose: 0 : [5576] Socket#27936619::Receive()
System.Net.Sockets Verbose: 0 : [5576] Data from Socket#27936619::Receive
System.Net.Sockets Verbose: 0 : [5576] 00000000 : 32 35 30 2D 64 6F 6E 6A-61 63 6F 62 73 61 75 74 : 250-xxxxxxxxxxxx
System.Net.Sockets Verbose: 0 : [5576] 00000010 : 6F 2E 6E 65 74 20 48 65-6C 6C 6F 20 41 65 66 32 : xxxxx Hello Aef2
System.Net.Sockets Verbose: 0 : [5576] 00000020 : 30 30 30 2E 61 75 74 6F-65 66 6F 72 6D 73 2E 62 : 000.autoeforms.b
System.Net.Sockets Verbose: 0 : [5576] 00000030 : 69 7A 20 5B 37 36 2E 31-32 2E 31 39 2E 31 32 5D : iz [11.11.11.11]
System.Net.Sockets Verbose: 0 : [5576] 00000040 : 2C 20 70 6C 65 61 73 65-64 20 74 6F 20 6D 65 65 : , pleased to mee
System.Net.Sockets Verbose: 0 : [5576] 00000050 : 74 20 79 6F 75 0D 0A 32-35 30 2D 45 4E 48 41 4E : t you..250-ENHAN
System.Net.Sockets Verbose: 0 : [5576] 00000060 : 43 45 44 53 54 41 54 55-53 43 4F 44 45 53 0D 0A : CEDSTATUSCODES..
System.Net.Sockets Verbose: 0 : [5576] 00000070 : 32 35 30 2D 50 49 50 45-4C 49 4E 49 4E 47 0D 0A : 250-PIPELINING..
System.Net.Sockets Verbose: 0 : [5576] 00000080 : 32 35 30 2D 38 42 49 54-4D 49 4D 45 0D 0A 32 35 : 250-8BITMIME..25
System.Net.Sockets Verbose: 0 : [5576] 00000090 : 30 2D 53 49 5A 45 20 32-35 37 32 38 36 34 30 0D : 0-SIZE 25728640.
System.Net.Sockets Verbose: 0 : [5576] 000000A0 : 0A 32 35 30 2D 44 53 4E-0D 0A 32 35 30 2D 45 54 : .250-DSN..250-ET
System.Net.Sockets Verbose: 0 : [5576] 000000B0 : 52 4E 0D 0A 32 35 30 2D-41 55 54 48 20 44 49 47 : RN..250-AUTH DIG
System.Net.Sockets Verbose: 0 : [5576] 000000C0 : 45 53 54 2D 4D 44 35 20-43 52 41 4D 2D 4D 44 35 : EST-MD5 CRAM-MD5
System.Net.Sockets Verbose: 0 : [5576] 000000D0 : 0D 0A 32 35 30 2D 44 45-4C 49 56 45 52 42 59 0D : ..250-DELIVERBY.
System.Net.Sockets Verbose: 0 : [5576] 000000E0 : 0A 32 35 30 20 48 45 4C-50 0D 0A                : .250 HELP..
System.Net.Sockets Verbose: 0 : [5576] Exiting Socket#27936619::Receive() 	-> 235#235
System.Net.Sockets Verbose: 0 : [5576] Socket#27936619::Send()
System.Net.Sockets Verbose: 0 : [5576] Data from Socket#27936619::Send
System.Net.Sockets Verbose: 0 : [5576] 00000000 : 4D 41 49 4C 20 46 52 4F-4D 3A 3C 44 45 4E 4E 49 : MAIL FROM:<DENNI
System.Net.Sockets Verbose: 0 : [5576] 00000010 : 53 40 44 4F 4E 4A 41 43-4F 42 53 54 4F 59 4F 54 : S@xxxxxxxxxxxxxx
System.Net.Sockets Verbose: 0 : [5576] 00000020 : 41 2E 43 4F 4D 3E 0D 0A-                        : A.COM>..
System.Net.Sockets Verbose: 0 : [5576] Exiting Socket#27936619::Send() 	-> 40#40
System.Net.Sockets Verbose: 0 : [5576] Socket#27936619::Receive()
System.Net.Sockets Verbose: 0 : [5576] Data from Socket#27936619::Receive
System.Net.Sockets Verbose: 0 : [5576] 00000000 : 32 35 30 20 32 2E 31 2E-30 20 3C 44 45 4E 4E 49 : 250 2.1.0 <DENNI
System.Net.Sockets Verbose: 0 : [5576] 00000010 : 53 40 44 4F 4E 4A 41 43-4F 42 53 54 4F 59 4F 54 : S@xxxxxxxxxxxxxx
System.Net.Sockets Verbose: 0 : [5576] 00000020 : 41 2E 43 4F 4D 3E 2E 2E-2E 20 53 65 6E 64 65 72 : A.COM>... Sender
System.Net.Sockets Verbose: 0 : [5576] 00000030 : 20 6F 6B 0D 0A                                  :  ok..
System.Net.Sockets Verbose: 0 : [5576] Exiting Socket#27936619::Receive() 	-> 53#53
System.Net.Sockets Verbose: 0 : [5576] Socket#27936619::Send()
System.Net.Sockets Verbose: 0 : [5576] Data from Socket#27936619::Send
System.Net.Sockets Verbose: 0 : [5576] 00000000 : 52 43 50 54 20 54 4F 3A-3C 62 66 6F 6C 65 79 32 : RCPT TO:<bfoley2
System.Net.Sockets Verbose: 0 : [5576] 00000010 : 33 35 40 68 6F 74 6D 61-69 6C 2E 63 6F 6D 3E 0D : 35@hotmail.com>.
System.Net.Sockets Verbose: 0 : [5576] 00000020 : 0A                                              : .
System.Net.Sockets Verbose: 0 : [5576] Exiting Socket#27936619::Send() 	-> 33#33
System.Net.Sockets Verbose: 0 : [5576] Socket#27936619::Receive()
System.Net.Sockets Verbose: 0 : [5576] Data from Socket#27936619::Receive
System.Net.Sockets Verbose: 0 : [5576] 00000000 : 35 35 30 20 35 2E 37 2E-31 20 3C 62 66 6F 6C 65 : 550 5.7.1 <bfole
System.Net.Sockets Verbose: 0 : [5576] 00000010 : 79 32 33 35 40 68 6F 74-6D 61 69 6C 2E 63 6F 6D : y235@hotmail.com
System.Net.Sockets Verbose: 0 : [5576] 00000020 : 3E 2E 2E 2E 20 52 65 6C-61 79 69 6E 67 20 64 65 : >... Relaying de
System.Net.Sockets Verbose: 0 : [5576] 00000030 : 6E 69 65 64 0D 0A                               : nied..
System.Net.Sockets Verbose: 0 : [5576] Exiting Socket#27936619::Receive() 	-> 54#54
System.Net Error: 0 : [5576] Exception in the SmtpClient#56956049::Send - Mailbox unavailable. The server response was: 5.7.1 <bfoley235@hotmail.com>... Relaying denied
System.Net Error: 0 : [5576]    at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception)
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
System.Net.Sockets Verbose: 0 : [5576] Socket#27936619::Dispose()
System.Net Verbose: 0 : [5576] Exiting SmtpClient#56956049::Send() 
System.Net.Sockets Verbose: 0 : [5576] Socket#30171786::Send()
System.Net.Sockets Verbose: 0 : [5576] Data from Socket#30171786::Send
System.Net.Sockets Verbose: 0 : [5576] 00000000 : 01 00 00 00 0E                                  : .....
System.Net.Sockets Verbose: 0 : [5576] Exiting Socket#30171786::Send() 	-> 5#5
System.Net.Sockets Verbose: 0 : [5576] Socket#30171786::Receive()
System.Net.Sockets Verbose: 0 : [5576] Data from Socket#30171786::Receive