To have your email account working with Hostinger you only need to do a few easy steps:
If you pointed domain by nameservers (NS records), all the required MX, SPF, and DKIM records will be added automatically.
Keep in mind that your subdomain needs a separate set of records
What to do if your domain is using external nameservers?
If your domain is pointing elsewhere by NS records, your DNS Zone management is moved to the provider you pointed the domain to and should be managed from there.
As long as your domain is pointing to Hostinger by A record, the changes you should do to your active DNS zone are:
Find default Hostinger Email DNS records at Emails → domain name → DNS settings:
Just click on the arrow near the record type:
After records are set, it may take up to 24 hours for them to propagate
When records finish propagating, don’t forget to verify them: