Email is annoyingly hard, if you want it to be reliable
I've setup mine to work with Mailgun and Firebase (for transactional emails)
This is easy
MX seankhliao.com mxa.mailgun.org
MX seankhliao.com mxb.mailgun.org
This is complicated to do safely and securely
Sender Policy Framework
Identifies which mail servers are allowed to send email from the domain
TXT seankhliao.com v=spf1 include:_spf.firebasemail.com include:mailgun.org ~all
Domain Keys Identied Mail
Email hashes are signed and verified with public-private keys,
public keys identified by the DKIM-Signature header,
...; s=selector; d=example.com corresonds to the key at selector._domainkey.example.com
CNAME firebase1._domainkey mail-seankhliao-com.dkim1._domainkey.firebasemail.com
CNAME firebase2._domainkey mail-seankhliao-com.dkim2._domainkey.firebasemail.com
TXT smtp._domainkey v=DKIM1; k=rsa; p=...
Domain-based Message Authentication, Reporting & Conformance
Specifies the policies (monitor, quarantine spam, reject) that should be applied when verifying against SPF and DKIM
Set another TXT some.doman._report._dmarc v=DMARC1 if you want to receive reports for somae.domain at antoher domain
TXT _dmarc v=DMARC1; p=reject; adkim=r; aspf=r; ruf=mailto:...; rua=mailto:...; fo=1; ri=86400; pct=100; rf=afrf;