Plesk - Fixing 550-5.7.26 do not pass when sending to Google Mail with external DNS Server

If you receive a message when sending an email to a Google account, something along the lines

This message does not pass authentication checks (SPF and DKIM
   both 550-5.7.26 do not pass).

From what I can tell, at least one of  these options has to pass in order to send emails to google  from you mail server. Lets have a look at some of the options supported with the latest version of Plesk.

SPF - Sender Policy Framework

If using Plesk's own DNS service this is likely already taken care for you.  You will likely find an entry in your domains DNS records along the  lines:

TXT	@	"v=spf1 +a +mx +a:<yourdommain.com>-all"

If  you are using a third party DNS service, in order to validate you domain when sending an email, chances are you will need to modify these DNS records to include this entry.

DKIM - DomainKeys Identified Mail

Use DKIM spam protection system to sign outgoing email messages you will likely need to configure DKIM DNS records on the DNS server yourself.

To do so, use the DKIM DNS records along the lines:

default._domainkey.<yourdomain.com>. IN TXT "v=DKIM1; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDxXCSUL34OoZ/aQBzHW+x1eGiFcUA8MTQZzo2wk8V4ESLyPZz/t0S7nb6a8XuGPq10jS/mt6+pUjGmBElFDcIh0wjCgaoSEj4/xEK4OdfsfxyKAPC0cqEs8dJV+3VV715kY/PnVFAGOCdLlmwptO2ttw0/l6vRsPDEwt6P8OoxG50wIDAQAB;"
_domainkey.yourdomain.com. IN TXT "o=-"

If you have domain aliases you use to send mail, add the DKIM DNS records for them as well. Use the same records as for the main domain but with the domain alias name.

DMARC -  Domain based message Authentication, Reporting and Conformance

Furthermore to also support (DMARC validation you can also add a TXT entry along the lines once you have enabled on the server.

_dmarc	"v=DMARC1; p=none"; adkim=r; aspf=r; fo=0; pct=100; ri=86400;

Summary

There are lots of Google email users out there. If you want to avoid your messages getting bounced back, there are a few methods and  technologies here that will help you to make  sure your email arrives at it's destination once you have correctly configured your server and respective DNS entries.