I've setup a whole new VPS according this howto. I use an external DNS. Everything works really nice, but there is an issue with my DNS TXT record at the DNS provider. The output of amavisd-new testkeys domain.tld gives the output: invalid (public key: not available). When I send an e-mail to a GMail account for example, I see the error code dkim=temperror (no key for signature) in the source of the e-mail. The format of the suggested DKIM DNS record, generated in ISPConfig for domain.tld does not seem to be ok as an DNS TXT record at my external DNS-provider. I tried the record with and without " ", but nothing seems to work. I still got invalid (public key: not available). What might be the solution for this? *UPDATE* Solved. ISPConfig3.1 with DKIM works out of the box. I had to change the host in DKIM TXT record at my external DNS provider from default._domainkey.domaint.tld into default._domainkey.