Error message after sending

When an e-mail is sent to a gmail.com address, the mail does not arrive at the received and the sender receives an error message in the following format:

<email@gmail.com>: host gmail-smtp-in.l.google.com[XXXX] said : 550-5.7.26 This mail is blocked because the sender is not verified. 550-5.7.26
Gmail requests that all senders are authenticated with SPF or DKIM.
550-5.7.26 550-5.7.26 Authentication results: 550-5.7.26 DKIM = failed 550-5.7.26 SPF [domain] with ip : [XXXX] = failed 550-5.7.26 550-5.7.26

Adding an SPF record.

This problem occurs when an email is sent from a domain name whose domain name zone (DNS configuration) does not contain a correct SPF (Sender Policy Framework) value.

The SPF record (a TXT record) tells the mail server (gmail.com in this case) whether the email sent by email@domainname was allowed to be be sent by the mail server.
If no record is present (or the SPF record is not valid) then gmail.com will detect this and reject the mail message.

Mails sent via Kinamo

If the mail is sent via a Kinamo mailserver (mail, webmail) then at least the SPF record should contain following values:

@ IN TXT "v=spf1 include:spf.kinamo.be ~all"

By adding include:spf.kinamo.be you are communicating to the receiving mail servers that the mail server infrastructure of Kinamo is allowed to send mail for your domain name.

It is possible that the SPF record - if present - already contains other values such as mailing services, outlook (Microsoft records) or others.