What error message are you receiving?

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:

 n.klokvandijk@gmail.com
   host gmail-smtp-in.l.google.com [2a00:1450:4025:401::1b]
   SMTP error from remote mail server after end of data:
   550-5.7.26 This message does not have authentication information or fails to
   550-5.7.26 pass authentication checks (SPF or DKIM). To best protect our users
   550-5.7.26 from spam, the message has been blocked. Please visit
   550-5.7.26  https://support.google.com/mail/answer/81126#authentication for more
   550 5.7.26 information. g16-20020a50ee10000000b0043580a9f835si1716275eds.494 - gsmtp

This is the answer I get.

Reporting-MTA: dns; mailrelay02.solcon.nl

Action: failed Final-Recipient: rfc822;n.klokvandijk@gmail.com Status: 5.0.0 Remote-MTA: dns; gmail-smtp-in.l.google.com Diagnostic-Code: smtp; 550-5.7.26 This message does not have authentication information or fails to

550-5.7.26 pass authentication checks (SPF or DKIM). To best protect our users
550-5.7.26 from spam, the message has been blocked. Please visit
550-5.7.26  https://support.google.com/mail/answer/81126#authentication for more
550 5.7.26 information. g16-20020a50ee10000000b0043580a9f835si1716275eds.494 - gsmtp

This suggest that your message is being flagged as spam by Google. Have you clicked on the link they specified?

This can happen if you are sending spam, or if your mail server is (probably shared, as many are) and the server is being used by others to send spam. There are many variations of the above that can also explain why. I've had this myself. I had to contact my hosting provider and they dealt with Goggle to get it reinstated as not spam. Someone else was using my shared server to send spam, and they terminated the other account.

When many servers (not just Google) receive an email, they first check the domain name of your email address and compare the the domain of the host that sent your email to see if there is a relationship. For example, if you were writing from a verizon account, but your email id did not reflect verizon, that would indicate possible spam, causing the message to be deleted. You might check with your email host on this. This is not a Thunderbird problem.