I’m using SMTPmart for bulk email marketing, but many emails are landing in spam instead of inboxes. I already added SPF, DKIM, and DMARC records correctly. What settings or best practices can improve email deliverability and inbox rate?
don't bother ,if you are planning to use mass mailing, use mailchimp or other providers for such campaigns
If you still want to do your own thing be sure your ip addresses have good reputation and "warmed up"