Unable to send email to Gmail: This message does not pass authentication checks (SPF and DKIM both 550-5.7.26 do not pass)

Symptoms

• Unable to send email from the mailbox of the example.com domain hosted in Plesk to Gmail with the error:

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

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  Reason: 550-5.7.26 This mail is unauthenticated, which poses a security risk to the 550-5.7.26 sender and Gmail users, and has been blocked. The sender must 550-5.7.26 authenticate with at least one of SPF or DKIM. For this message, 550-5.7.26 DKIM checks did not pass and SPF check for [example.com] 550-5.7.26 did not pass with ip: [203.0.13.2]. The sender should visit 550-5.7.26 https://support.google.com/mail/answer/81126#authentication for 550 5.7.26 instructions on setting up authentication.

• The DNS zone of the exmaple.com domain is hosted externally 

Cause

SPF and DKIM records are missing or incorrect for the example.com domain on DNS server or DNS Registrar side.

Resolution

1. Enable DKIM and SPF
2. Go to Domains > example.com > Hosting & DNS > DNS Settings page and find DKIM and SPF records
3. Copy DKIM and SPF record from Plesk to external DNS server which servesexample.comzone and to DNS registrar example.com was purchased from