Hello @ClaireE078 ,
Hello, I'd recommend checking out this article on general ways to improve and maintain higher deliverability. With that said, there's several reasons why your emails may be ending up in some contacts' junk folders. This can depend significantly on the settings of your contact's email client, as well as any security elements they or their IT teams may have set as well. Safelisting is something that can be done on the contact's end to make sure your emails aren't being unduly forced into spam or completely mitigated from receiving.
While all emails sent through us have some degree of authentication, some email clients can be very particular about the source of emails being sent. I'd recommend looking into authentication, either through us if you're using a free email service (Gmail, Yahoo, Live, etc.) or self-authentication if you have your own domain.
It'd also likely be worth calling and speaking with our Deliverability team, to see if they have any additional specialized insight they can offer.