Inbox providers check whether a sending domain has authorized the sender. The DNS records shown for your custom email domain provide that authorization; missing or incorrect records can make messages look untrusted.
SPF, DKIM, and DMARC
SPF identifies the mail servers allowed to send for your domain. DKIM lets receiving servers verify a message's signature. DMARC tells receiving servers how to handle messages that fail authentication checks.
If delivery quality drops
Check that every DNS record shown in Email settings is still present at your domain provider, especially after a DNS migration or provider change. Review failed and bounced messages in Send history, and keep promotional email on a marketing sender address.