Google servers, whether Gmail or company mailboxes hosted by Google have been rejecting each and every e-mail that I sent for the past week or so. There are no indications as to why the message was rejected and I can't figure out what is wrong. I am now at the end of my wits.
Here is the rejection message:
host gmail-smtp-in.l.google.com[2a00:1450:400c:c07::1b] said: 550-5.7.1 [2001:41d0:52:100::1161 12] Our system has detected that this 550-5.7.1 message is likely unsolicited mail. To reduce the amount of spam sent 550-5.7.1 to Gmail, this message has been blocked. Please visit 550-5.7.1 https://support.google.com/mail/?p=UnsolicitedMessageError 550 5.7.1 for more information. n30si13214745wrb.62 - gsmtp (in reply to end of DATA command)
The sender is a VPS that I own and administer myself, for my one-person company, running CentOS 6, ISPConfig, amavisd-new, ClamAV and SpamAssassin. I have every standard antispam countermeasures that I know of:
- reverse DNS (both IPv4 and IPv6) entries matching the MX entries.
Except for DMARC, which is a new thing (I've added the DNS record just today), the rest has been set up for years.
Every e-mail that I send is legitimate and I can't understand why I can't even reply to an e-mail sent to me, nor why all of that didn't protect me from such an inconvenience.
What I've tried so far
• I have checked the blacklisting status of my domain with mxtoolbox.com: it was blacklisted in two of the thirty blacklists the site helps check. I have had it delisted from one and things seem more complicated on the second, SORBS, which has (drumroll...) one single suspicious e-mail: my server is now blacklisted because some software program somewhere thought that one e-mail was suspicious.
• I have used the e-mail server test at allaboutspam and whenever I have my signature in the test e-mail, it says the e-mail contains a URL blacklisted in black.uribl.com but uribl.com says it doesn't list my domain. That's quite confusing.
• I have sent the same message to my e-mail accounts (TO'ed, not CC'ed) with Yahoo, Outlook and GMail: Yahoo accepted the message, Outlook put it in the Junk folder and GMail downright rejected it.
What other actions can I take to certify and authenticate my legitimate e-mails?
Last, I wonder why there is a
12 after the condensed IPv6 address. Is it a code of some sort that could guide me to the solution?