Honestly, I don't think the SMTP server alone is the reason your emails are going to spam. I've used SMTPmart's SMTP Server for bulk campaigns, and while the deliverability has been solid, Gmail cares a lot more about sender reputation, list quality, engagement rates, and the content of your emails.

If SPF, DKIM, and DMARC are already set up correctly, I'd check whether your domain has been properly warmed up and whether you're sending to a clean, engaged list. Even AWS SES can end up in spam if the domain reputation isn't strong enough.

From my experience, SMTPmart has been reliable, but improving inbox placement usually comes down to the overall sending practices rather than switching SMTP providers.