All you need to do is to go to DNS Zone of your domain and add the following MX records:
mx1.flockmail.com
mx2.flockmail.com
And these TXT records:
v=spf1 include:spf.flockmail.com include:spf.mx.hostinger.com include:relay.mailchannels.net ~all
v=DMARC1;p=none;sp=none;adkim=r;aspf=r;pct=100
