I use Hover for my domain registration and email. I couldn’t send emails to Gmail addresses, as they were all bouncing back.

<john.doe@gmail.com>: host gmail-smtp-in.l.google.com[***.***.***.**] said:
    550-5.7.26 This mail has been blocked because the sender is
    unauthenticated. 550-5.7.26 Gmail requires all senders to authenticate with
    either SPF or DKIM. 550-5.7.26  550-5.7.26  Authentication results:
    550-5.7.26  DKIM = did not pass 550-5.7.26  SPF [lucaf.eu] with ip:
    [***.***.***.**] = did not pass 550-5.7.26  550-5.7.26  To mitigate this
    issue, please visit Gmail's authentication guide 550-5.7.26 for
    instructions on setting up authentication: 550 5.7.26
    k33-**************************.*** - gsmtp (in reply to
    end of DATA command)

The error 5.7.26 is caused by Gmail’s Sender Policy Framework (SPF) requirement, which is not created by default for Hover’s email service. To fix the error, follow this guide and create a SPF record for your domain.

The record suggested in Hover’s guide fixed the issue for me.

v=spf1 include:_spf.hostedemail.com include:hover.com ~all