How you acquire your email addresses is the most important factor in delivering your emails to the inbox.
The Importance of Growing your Contact List Organically

Spam filtering is highly dependent on the email content, subject and from address.

We recommend the following to improve your deliverability:

Turn on Email Authentication in your Email Campaign configuration

One way that ISPs like Yahoo, Gmail, AOL, etc. fight spam is to use something called email authentication to check whether an email that claims it’s from you, is actually from you. If you’re using an @yahoo.com or @aol.com email address as your FROM address you’re especially at risk. AOL and Yahoo will reject the email outright if it’s sent to them and will not authenticate it went sent to other email providers. Yahoo! and AOL currently feature strict authentication policies, but it’s likely that other email providers will do the same in the future—possibly without warning.

Do not use a free email address for your FROM email address such as Gmail, Yahoo, Hotmail.
You have a higher chance of your email campaign reaching the inbox if you don’t use a free email address.
While it may appear to the subscriber that your email has arrived from one of these free webmail addresses, the recipient mail server will see that it came via another system and mark it as suspicious, potentially preventing it from reaching its destination.

Setting up a personal business domain is simple and cheap.

Setting up a business email address and domain will improve your deliverability this will help you gain a better sending reputation.

Google domains offers email forwards included with domain registration.
https://domains.google/learn/how-to-use-email-forwarding/

Use our Spam Checker on Step 2 – Content of the Email Studio.

Our spam checker is based on the spam assassin (http://spamassassin.apache.org) rule set and will screen for common spam keywords and ALL CAPS subjects such as “Viagra” or “YOU’RE A WINNER”.   It cannot duplicate any particular email recipient’s spam filters.

After checking that you are using recommended best practices for sending an email and the email is still
going to your SPAM folder you would need to contact your email provider to find out why their spam
rules are labeling it as spam.

Only send to contacts that have opted-in to receive your messages

Your from address domain has a reputation with every contact that you are sending to. Sending to emails that didn’t opt-in to receive emails from you can lead to high complaint levels and low engagement. Ultimately this can cause emails to start being delivered into spam and junk folders.

Please see our guide on the importance of growing your contact list organically.