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Mail server reports “550 Address unknown”, but check via telnet finds the address

Hello,

I'm trying to send an email through Postfix from mydomain.com to an address I know exists (info@example.org) - the receiving end seems to be running a Sophos product.
Postfix is reporting that the address does not exist.

Excerpt from my mail log

Jun 7 07:01:47 mydomain postfix/smtp[50352]: 010BB40606: to=<info@example.org>, relay=remote.example.org[redacted]:25, delay=1.2, delays=0.37/0/0.37/0.48, dsn=5.0.0, status=bounced (host remote.example.org[redacted] said: 550 Address unknown (in reply to RCPT TO command))

Here is the telnet session (from the server where Postfix runs) showing that the address exists:

# telnet remote.example.org 25
Trying [redacted]...
Connected to remote.example.org.
Escape character is '^]'.
220 sophos.example.local ESMTP ready.
HELO mydomain.com
250 sophos.example.local Hello mydomain.com [redacted]
MAIL FROM: <myaddress@mydomain.com>
250 OK
RCPT TO: <info@example.org>
250 Accepted

To me it looks like my email is rejected by Sophos for some reason. From what limited information I could gather so far, it seems to be related to "recipient verification". 

Could someone help me understand what kind of check the other side is running that keeps my email from being delivered? The email address I'm sending from and the address in the return path do exist (which can be checked by using telnet). 

Thanks!



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  • Hi,

    your first attemp to send the email is "bounced".. its a normal protection agains spammers.

    da ich denke du bist deutsch:

    https://de.wikipedia.org/wiki/Bounce_Message

     

    greets

    zaphod
    ___________________________________________

    Home: Zotac CI321 (8GB RAM / 120GB SSD)  with latest Sophos UTM
    Work: 2 SG430 Cluster / many other models like SG105/SG115/SG135/SG135w/...

  • I've tried it several times and the email bounces each time.

    For spam protection messages sometimes are greylisted and get the status "deferred", they are then retried later on. This seems to be not the case here, the message is bounced and will thus not be send again unless the user initiates a new mail.

  • bounce is a simple protection agains spammers... first mail from a source-mail server will be bounced... cause per default all mail servers will send a message after some time again...

    its a technical trick to get out the spams from dial-up host.. they will not retry to send...

    mailservers should send mails to other mailservers... users should use some internal mail servers or servers they have authenticated to so the server will then never bounce such a mail...

     

    greets

    zaphod
    ___________________________________________

    Home: Zotac CI321 (8GB RAM / 120GB SSD)  with latest Sophos UTM
    Work: 2 SG430 Cluster / many other models like SG105/SG115/SG135/SG135w/...

Reply
  • bounce is a simple protection agains spammers... first mail from a source-mail server will be bounced... cause per default all mail servers will send a message after some time again...

    its a technical trick to get out the spams from dial-up host.. they will not retry to send...

    mailservers should send mails to other mailservers... users should use some internal mail servers or servers they have authenticated to so the server will then never bounce such a mail...

     

    greets

    zaphod
    ___________________________________________

    Home: Zotac CI321 (8GB RAM / 120GB SSD)  with latest Sophos UTM
    Work: 2 SG430 Cluster / many other models like SG105/SG115/SG135/SG135w/...

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