GMail is changing its SPAM policy

You will receive emails from customer email addresses with content similar to:

Reference: Mail delivery failed: returning message to sender

• This message was created automatically by mail delivery software.
  Error
  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:
•  SMTP error from remote mail server after end of data:
   123-4.5.67 This message does not pass authentication checks (SPF and DKIM both
   123-4.5.67 do not pass). SPF check for [testmail@Teststudio.de] does not pass with
   123-4.5.67 ip: [12.345.678.9101].To best protect our users from spam, the
   123-4.5.67 message has been blocked.

Gmail has changed and tightened its SPAM policy.
So what you have to do from now on is one SPF (stands for Sender Policy Framework, i.e. “safe sender”) record in your domain, so add on your website. It basically serves to protect against SPAM.

Here you can find more information about this and how this record can be added:

> SPF record at the mail provider

You can find answers and information if you search for your email provider and enter the word "SPF" in the Google search

e.g.

Hosteurope:
https://www.hosteurope.de/faq/domains/verwaltung/spf

Keywords: mail provider, email, outbox, email delivery, mail cannot be delivered, sent, email delivery, Gmail, no email delivery possible, no appointment reminder