TLDR; When sending email to a new person, make sure the email is more than just a sentence or two, and that it has some actual content. Otherwise, it may be marked as spam.
I just got a new Outlook 365 email account for a new job. When I emailed myself 3 one-sentence emails from my new account to my home email account as a test, things like “Testing!” and “Did this make it to Lee? asdf” they all fell into the spam folder on my paid Google Workspace account. When I sent longer emails, they stopped going to spam. Even new, short emails stopped falling into spam. I assume that was my Gmail being extra-cautious about receiving phishing emails from a new email address.