I suspect this is a naive question but I'm at a loss how to fix this.

I send emails using thunderbird and msmtp. I post via my ISP's smtp server  (smpt.zen.co.uk)

This has allowed me to send emails to any one as any one. e.g. this has come from alug@nevilley.org.uk

very recently emails sent to gmail users have started to bounce undelivered with the likes of: (email address redacted)

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:

##############@gmail.com host gmail-smtp-in.l.google.com [108.177.15.27] SMTP error from remote mail server after pipelined end of data: 550-5.7.26 This message does not pass authentication checks (SPF and DKIM both 550-5.7.26 do not pass). SPF check for [nevilley.org.uk] does not pass with ip: 550-5.7.26 [212.23.1.22].To best protect our users from spam, the message has 550-5.7.26 been blocked. Please visit 550-5.7.26https://support.google.com/mail/answer/81126#authentication for more 550 5.7.26 information. p9-20020adfe609000000b002ca647fe919si1349235wrm.527 - gsmtp

Do I need to do something with thunderbird on my PC or is this a problem for Zen? Any help gratefully received. Nev