Any question answered...

Any ideas what I can do to fix this - I have an email service / domain with Clara Soho, more or less a relic of early days of internet but I like to keep the email addresses this provides. Recently I get my emailed bounced when sending on to Gmail addresses with the following error:

host gmail-smtp-in.l.google.com [66.102.1.27]

SMTP error from remote mail server after pipelined end of data:
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