When you send a 1:1 email or sales emails from HubSpot, it isn't actually HubSpot sending the email. Rather, HubSpot sends a trigger to your external email provider and then sends the email.
This blog offers some tips to prevent your emails from going to spam. It discusses some tips such as don't use all caps in the email's subject line and offers a list of spam keywords. https://blog.hubspot.com/blog/tabid/6307/bid/30594/a-marketer-s-guide-to-getting-past-email-spam-fil...Since one-to-one emails are sent by your external email client, there is limited troubleshooting that can be done from HubSpot's side; however, I recommend following some of the tips in the blog post to make sure the wording or format of your email is causing it to go to spam.
I'll also tag a few experts that can share their experiences.
I agree with @sharonlicari on this one. Just using a template in a 1:1 email scenario isn't going to result in being blocked by spam filters. The resource that she provided is a great one to follow when building these templates for the sales team. I understand their hesitation, but with some knowledge transfer, you should be able to assure them that templates will be more advantageous than harmful.
As an additional tip, just remind them that a template is a template. A great sales email should also use some personalization aspect to it so it truly is 1:1.
Best of luck!
Josh
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Josh Curcio HubSpot support and inbound marketing for OEMs, contract manufacturers, and industrial suppliers. HubSpot Platinum Partner & HubSpot Certified Trainer