Hello @Jozef,
Do you use the Contact form 7 plugin? Read next articles – https://contactform7.com/configuration-errors/email-not-in-site-domain/ https://wpmailsmtp.com/contact-form-7-not-sending-emails/#:~:text=The%20error%20%E2%80%9CSender%20email%20address,the%20message%20is%20sent%20from.
Regards
