Hello, 

my company has a problem with sending emails to our customers. 

We received a few messages with the following content: 

"This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error.

xxx @muzeum.torun.pl INVALID_ADDRESS, ERROR_CODE :554, ERROR_CODE :5.7.1 Service unavailable; Client host [135.84.80.218] blocked using dnsbl.sorbs.net; Currently Sending Spam See: http://www.sorbs.net/lookup.shtml?135.84.80.218

    with SMTP id 1541764765268560.2189093290793; Fri, 9 Nov 2018 03:59:25 -0800 (PST)

Message-ID:<166f8575501.d7167643118606 .2456340647536212860@narysujto.pl>

Date:Fri, 09 Nov 2018 12:59:24 +0100

From:Szymon Berbeka <xxxx @narysujto.pl>

User-Agent:Zoho Mail

To:<xxxx @muzeum.torun.pl>

Subject:Przetarg

Content-Type:multipart/mixed;

    boundary="----=_Part_361605_1277881113.1541764764929"

"

Unfortunately, we've missed the deal with potential client... for 20k net

How we can solve this issue? 

Best regards

Robert Kosak