Email NOT sent to Gmail with Netlify DNS

Hi everyone! I have webhosting and email from Netcup and have created a website which I've hosted on Netlify http://www.cesaro-bau.de.

Email has been working fine for some months but now I receive a mail failure when emailing to Gmail as such:

I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.

said: 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 [cesaro-bau.de] did not 550-5.7.26 pass with ip:

I've pointed the DNS in Netcup to Netlify's and have the following DNS added in Netlify:

cesaro-bau.de 10 in MX 10 mail.cesaro-bau.de

cesaro-bau.de 50 in MX 50 mxe815.netcup.net

cesaro-bau.de 3600 in Netlify cesaro.netlify.app

http://www.cesaro-bau.de 3600 in Neltify cesaro.netlify.app

what else should I add to make SPF and DKIM work?

Thank you!

many thanks! the " " for TXT certificate are not to be used, right?