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Since several months I have sporadic problems when I send emails.

With tcpdump I finally found out that I'm getting "Port unreachable" messages from what seems to be my email provider, according to the source IP. But my email provider says they cannot find a problem in their network and say it must be in my network. To exclude problems from the firewall I setup port mirroring and captured the packets on the port connected to my cable modem which is operating as a bridge.

I did a lot of troubleshooting already. I see:

  • the errors happen both when I send from my Android phone on Wi-Fi or from Thunderbird on my wired PC.
  • the errors happen when I send to the email-provider to SMTP port 465 or 587
  • it's not happening always; after some tries finally the mail gets send; or it is sent at the first try
  • I just saw that I'm also getting "Port Unreachable" sent by my DNS resolver to the forwarding DNS Server (1.1.1.1, 1.0.0.1) when receiving DNS responses. I guess this is unrelated to the email problem, but who knows?

Here is one example of the ICMP error I'm getting.

Internet Control Message Protocol
    Type: 3 (Destination unreachable)
    Code: 3 (Port unreachable)
    Checksum: 0x8c59 [correct]
    [Checksum Status: Good]
    Unused: 00000000
    Internet Protocol Version 4, Src: <my-ip>, Dst: <smtp-server-ip>
    Transmission Control Protocol, Src Port: 48624, Dst Port: 587, Seq: 3998125229
        Source Port: 48624
        Destination Port: 587
        Sequence number: 3998125229
        [Stream index: 0]
        Acknowledgment number: 0
        Acknowledgment number (raw): 0
        1010 .... = Header Length: 40 bytes (10)
        Flags: 0x002 (SYN)
        Window size value: 29200
        [Calculated window size: 29200]
        Checksum: 0x3680 [unverified]
        [Checksum Status: Unverified]
        Urgent pointer: 0
        Options: (20 bytes), Maximum segment size, SACK permitted, Timestamps, No-Operation (NOP), Window scale
        [Timestamps]

What can go wrong here, and at which part of the network? Is it plausible, that the problem is neither in my network nor in the email provider's network?

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    What TTL do these ICMP error packets have? How does it compare to legitimate responses from your mail server? – user1686 Feb 7 at 17:00
  • @user1686 The ICMP packet in my question above has a TTL of 51 (captured before going through my firewall). A legitimate SMTP packet coming from the server has a TTL of 50 captured behind my firewall. Do you want to find out, assuming all the packets take the same path, both (ICMP and SMTP reply) are coming from the same device? – bug.spencor Feb 7 at 18:25
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My email provider is blocking access from specific IPs if there are multiple failed login attempts from that address during a specific time range to avoid abuse.

Turned out that one of my machines was causing those failed logins due to TLS errors, so my IP address got blocked temporarily. Turning of these logins resolved the issues.

It was really hard troubleshooting that one. The provider did not give details about why SMTP failed for me. What helped solving it in the end was sniffing/analyzing traffic from my whole network to the provider's hosts.

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