​First setup your custom sending domain for email 

Check here how to configure the emails that we send on your behalf https://help.firmhouse.com/en/articles/3008902-changing-the-sent-from-and-reply-to-email-addresses-for-your-emails

To improve the email delivery of the notifications that Firmhouse sends from your custom domain, these DNS records need to be applied: SPF, DKIM & DMARC.

When the sending email address is notification@subs.example.com, the following changes are needed

is a TXT record on the subs.example.com domain
we will provide you with the value of that TXT record

Is also a TXT record of which we will provide the name and value, will probably look like default._domainkey.subs.example.com

This record, also a TXT record, will have to added on _dmarc.subs.example.com .
Most basic value should be v=DMARC1; p=none;
​You can also set up to receive delivery reporting, for more info on this check https://www.mailhardener.com/kb/dmarc