TL;DR

Set the following two entries for your domain:

And your domain can't be misused for spam or bad mails anymore.

Explanation

SPF

What is SPF for?

SPF allows you to set some rules for emails, with your domain in the From: field of an email, are allowed to come from.

This prevents cases where a random person just yoinks your domain and put it in the From: field of their mail and abusive it for spam mails for example.

Content description

v=spf1 -all means:

• v=spf1: We are using the 1st version of SPF
• -all: This domain doesn't send any mail at all.".

So if others are getting mails from your domain, they can see that this can't be possible, since you're saying here that no mails can be send from your domain, hence the receiver can happily discard it for example.

For a more detailed explaination, click here.

DMARC

What is DMARC for?

With DMARC you can tell others which policy (that's what the p= means) should be used if any checks failed.

The checks are usually from SPF and DKIM (which allows you to sign your mails) but if you don't want to use your domain to send mails anyhow, then SPF should be enough.

Content description

v=DMARC1; p=reject; means here:

• v=DMARC1: Use the first version of DMARC
• p=reject;: Just reject the mail and do nothing.

If you want to read more about DMARC, I think this is a good page.