SMTP error: 550 5.7.1 IP listed on RBL (SBLCSS)

My wife replied to an e-mail and got the following error back:

<>: host[2a01:238:20a:202:50f0::1097]
said: 550 5.7.1 IP listed on RBL ( (in reply to RCPT TO command)

I immediately checked if my server was on blacklists. MXToolbox's blacklist check did not show any errors, and my own is-my-server-sending-too-many-mails check had not triggered.

The error message indicated that IPv6 was used, and Spamhaus's CSS FAQ says that they only list /64 subnets, and not individual IPv6 addresses - the database would simply get too large.

Now our server hoster Hosteurope only gives us a /128 block, which is a single IPv6 address. This means that another server hosted there sent spam mails, got blacklisted and now my server is punished, because we share the same /64 block :(

I contacted Hosteurope's support, but they told me that IPv6 is still in beta for them and that they could not give me my own /64 range. They also told me I should send my mail via IPv4 :(

I then configured postfix to prefer IPv4 over IPv6 (man 5 postconf):

smtp_address_preference = ipv4

Written by Christian Weiske.

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