Mail Delivery failed - what to do

I can receive Emails with K-9, but when I reply, I always get a notice, that my mail delivery failed. "there is a permanent error. SMTP error from remote mail server after pipelined end of data. 550-5.7.26 This mail is unauthenticate, 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 … What does that mean?

It means that your mail server has neither an authentic sender claim (SPF) nor does it indicate from which IPs your domain send mail (DKIM). – Gmail flags mail from untrusted senders. If SPF and DKIM lookup fail, the flagged mail is rejected. It’s a spam protection measure.

Contact your provider and ask them why they have not set up SPF and DKIM, and ask when they intend to fix the issue.

If you admin yourself, make sure your MX is set up properly and then add the SPF and DKIM record to your DNS record.

I am sorry, but I don’t understand. Who do you mean with provider? Hostpoint maybe ? (that’s where I bought my personalized email adress)…

Well, in that case yes, they are your provider. Try following their DMARC FAQ: How can I add an SPF Record for my domain?

MX records are for inbound mail connections, so not relevant here. what you need, at minimum, is an SPF record for the domain/sub-domain you are sending from (on the From: on your messages) that includes the IPnumber(s) or resolved names of the mail server(s) your mail goes out from. at this point most mail service providers have a cheatsheet with what you need to add to your domain’s dns record for their mail servers.