yes definitely test your emails using
https://www.mail-tester.com/
what I found being useful in lowering spam score is modifying your activation email so that it is unique - write a short welcome message linked to your board
it is as easy as editing a plain text file in:
\language\en\email\user_welcome_inactive.txt
*(use your language instead of 'en' if applicable)
the downside is that this file will be replaced after each version update and you'll need to reapply this change
I did this and ca. 3-5% of registered users do not activate their accounts (whether due to activation email landing in spam or other reasons)