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I am having trouble sending mail from Linux to Exchange server. When I telnet from Linux to Windows on port 25, I am not receiving any message from Exchange. The connection is automatically terminated after few seconds of wait.

Using tcpdump I can see that Windows keeps sending SYN+ACK and finally closing connection. Even though Linux responds with ACK message, it is not seen by Windows. If I send some message (telnet test) without waiting for Exchange 'greeting', the connection works.

I had verified Windows firewall and connection from Linux machine is allowed and I cannot find any packets dropped message in firewall logs.

Connection from Windows to Linux port 25 works fine.

Details:

Windows: x.x.x.243 / 255.255.255.128; gateway = x.x.x.129

Linux: x.x.x.145 / 255.255.255.128; gateway = x.x.x.129

Linux to Windows tcpdump

root@yy01:~# tcpdump -nni eth0 host x.x.x.243 and port 25 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes 05:03:58.846266 IP x.x.x.145.45104 > x.x.x.243.25: Flags [S], seq 3228055127, win 29200, options [mss 1000,sackOK,TS val 659649138 ecr 0,nop,wscale 7], length 0 05:03:58.849071 IP x.x.x.243.25 > x.x.x.145.45104: Flags [S.], seq 37978062, ack 3228055128, win 8192, options [mss 1460,nop,wscale 8,sackOK,TS val 316999749 ecr 659649138], length 0 05:03:58.849097 IP x.x.x.145.45104 > x.x.x.243.25: Flags [.], ack 1, win 229, options [nop,nop,TS val 659649139 ecr 316999749], length 0 05:04:01.858960 IP x.x.x.243.25 > x.x.x.145.45104: Flags [S.], seq 37978062, ack 3228055128, win 8192, options [mss 1460,nop,wscale 8,sackOK,TS val 317000050 ecr 659649138], length 0 05:04:01.859001 IP x.x.x.145.45104 > x.x.x.243.25: Flags [.], ack 1, win 229, options [nop,nop,TS val 659649891 ecr 316999749], length 0 05:04:07.874811 IP x.x.x.243.25 > x.x.x.145.45104: Flags [S.], seq 37978062, ack 3228055128, win 65535, options [mss 1460,sackOK,TS val 317000651 ecr 659649138], length 0 05:04:07.874872 IP x.x.x.145.45104 > x.x.x.243.25: Flags [.], ack 1, win 229, options [nop,nop,TS val 659651395 ecr 316999749], length 0 05:04:19.875384 IP x.x.x.243.25 > x.x.x.145.45104: Flags [R], seq 37978063, win 0, length 0

Linux to Windows tcpdump with some data send without waiting for greeting

root@yy01:~# tcpdump -nni eth0 host x.x.x.243 and port 25 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes 05:06:30.521152 IP x.x.x.243.25 > x.x.x.145.45234: Flags [R], seq 1489851708, win 0, length 0 05:06:31.253677 IP x.x.x.145.45246 > x.x.x.243.25: Flags [S], seq 3514263924, win 29200, options [mss 1000,sackOK,TS val 659687240 ecr 0,nop,wscale 7], leng th 0 05:06:31.256475 IP x.x.x.243.25 > x.x.x.145.45246: Flags [S.], seq 3020263353, ack 3514263925, win 8192, options [mss 1460,nop,wscale 8,sackOK,TS val 317014 989 ecr 659687240], length 0 05:06:31.256520 IP x.x.x.145.45246 > x.x.x.243.25: Flags [.], ack 1, win 229, options [nop,nop,TS val 659687241 ecr 317014989], length 0 05:06:34.255733 IP x.x.x.243.25 > x.x.x.145.45246: Flags [S.], seq 3020263353, ack 3514263925, win 8192, options [mss 1460,nop,wscale 8,sackOK,TS val 317015 289 ecr 659687240], length 0 05:06:34.255782 IP x.x.x.145.45246 > x.x.x.243.25: Flags [.], ack 1, win 229, options [nop,nop,TS val 659687990 ecr 317014989], length 0 05:06:36.720344 IP x.x.x.145.45246 > x.x.x.243.25: Flags [P.], seq 1:3, ack 1, win 229, options [nop,nop,TS val 659688607 ecr 317014989], length 2: SMTP: 05:06:36.924198 IP x.x.x.145.45246 > x.x.x.243.25: Flags [P.], seq 1:3, ack 1, win 229, options [nop,nop,TS val 659688658 ecr 317014989], length 2: SMTP: 05:06:37.176212 IP x.x.x.145.45246 > x.x.x.243.25: Flags [P.], seq 1:3, ack 1, win 229, options [nop,nop,TS val 659688721 ecr 317014989], length 2: SMTP: 05:06:37.680210 IP x.x.x.145.45246 > x.x.x.243.25: Flags [P.], seq 1:3, ack 1, win 229, options [nop,nop,TS val 659688847 ecr 317014989], length 2: SMTP: 05:06:38.688207 IP x.x.x.145.45246 > x.x.x.243.25: Flags [P.], seq 1:3, ack 1, win 229, options [nop,nop,TS val 659689099 ecr 317014989], length 2: SMTP: 05:06:40.271716 IP x.x.x.243.25 > x.x.x.145.45246: Flags [S.], seq 3020263353, ack 3514263925, win 65535, options [mss 1460,sackOK,TS val 317015890 ecr 659687240], length 0 05:06:40.271765 IP x.x.x.145.45246 > x.x.x.243.25: Flags [.], ack 1, win 229, options [nop,nop,TS val 659689494 ecr 317014989], length 0 05:06:40.708211 IP x.x.x.145.45246 > x.x.x.243.25: Flags [P.], seq 1:3, ack 1, win 229, options [nop,nop,TS val 659689604 ecr 317014989], length 2: SMTP: 05:06:44.748217 IP x.x.x.145.45246 > x.x.x.243.25: Flags [P.], seq 1:3, ack 1, win 229, options [nop,nop,TS val 659690614 ecr 317014989], length 2: SMTP: 05:06:44.751495 IP x.x.x.243.25 > x.x.x.145.45246: Flags [P.], seq 1:93, ack 3, win 65533, options [nop,nop,TS val 317016338 ecr 659690614], length 92: SMTP: 220 EXxx.yyyy.local Microsoft ESMTP MAIL Service ready at Thu, 4 Feb 2016 05:06:44 +0100

Please advise on possible cause of this issue.

NB: I had asked same question on stackoverflow and they advised to ask here.

Thank you,

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  • 1
    Do you have a firewall on the Linux machine? iptables -L -n -v Commented Feb 4, 2016 at 19:29
  • Verified firewall firewall rules in Linux, nothing blocking there. As a work around we added additional IP to Linux (different network range) and added route to redirect traffic to x.x.x.243 through new IP and it works fine. The problem is between x.x.x.145 and x.x.x.243 IPs. We are still trying to find the root cause. Commented Feb 5, 2016 at 2:09

1 Answer 1

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configure your receive connector in exchange management console(EMC)>server config>hub transport>"name of your receive connector"

to accept communication on 25, set your local addresses to recieve mail from all v4 IPs, on port 25 and your remote servers that has your IP on the linux machine: x.x.x.145

for basic testing use auth TLS and permissions anonymous.

fqdn set your public domain name.(DNS A record that fqdn should point to your windows IP: x.x.x.243)

one more thing during testing - on the first tab of the recieve connector set protocol logging level to verbose.

then you can easily check what is going on on the windows side. logs are located in: C:\Program Files\Microsoft\Exchange Server\V14\TransportRoles\Logs\ProtocolLog\SmtpReceive

for testing purposes then use: telnet x.x.x.243 25

you should be able at least to connect to the server

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