DKIM is one of useful thing to make email which is sent by your domain to be avoided marked as spam.
When you use cloudflare for that domain, there is some change that you need to do on the DKIM key.
For example on your WHM (Edit DNS Zone) or cPanel (Email > Authentication), you will get DKIM key with format like this :
"v=DKIM1; k=rsa; p=XXXX" YYY\;
That format is invalid if you want to use Cloudflare DNS editor.
To add correct DKIM key, add a TXT record in your Cloudflare DNS editor :
Name : default._domainkey
v=DKIM1; k=rsa; p=XXXXYYY
You may notice that the double quotes, backslash, a space and a semicolon are removed.