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I had a several Undelivered Mail Returned to Sender with my domain. This started happening at least 2 months, after I made an HTML signature.

It is possible, I don't know very well the sense of this. My e-mail signature only as a link with my site name and a PNG of brand logo (13kb). So if you guys could help me with this I will be very gratful.

This is the returned to sender (the most usual):

DOMAIN is blacklisted on Spamhaus - see
    (in reply to RCPT TO command)

I checked my domain on their site and is not listed (idiot).

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up vote 2 down vote accepted

The message you're seeing is in regards to The Spamhaus Project, which compiles and distributes lists of IP's and domains associated with email spam so that email servers can filter them out.

Unlike content-matching tools like SpamAssassin, which uses a variety of spam-detection techniques (including analyzing the content of the message), filtering based on DNS-based Blocklists (DNSBLs) as provided by Spamhaus is based on IP or domain instead of the message body or signature therein.

Looking at the SMTP transaction, the response (in reply to RCPT TO command) seems to indicate that the recipient(s) domain is blacklisted on Spamhaus, so I would check the domains of the email addresses that you're sending to with the Spamhaus Domain Lookup Tool.

It might also be a good idea to check the domain and IP address that your sending from and the SMTP server you're using if you're getting other rejection responses.

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