SendGrid doesn't really send emails to the SPAM folder. Instead, it is up to the receiver's spam filters in the mail delivery flow.
A significant proportion of mails are tagged as spam for one of the reasons listed below.
People are labeling emails as spam because they are not adequately verified.
The message is not authenticated properly.
An email comes from an IP address that is blacklisted (perhaps SendGrid's).
Your email contains known phishing techniques (coincidentally or not).
Here's How To Resolve It
Try sending a message to yourself and open the message headers, then look for entries like the one below:
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Received: from xxxxxxxxx.com (xxxxxxx.com [xxx.xx.xxx.xx])
by xxxx.com with SMTPS id ##############
The IP address within [] is the IP address of the sending server. First, check a public blacklist, like spamhaus, to verify whether it's blacklisted, then report any matches to SendGrid.
Build your own email list.
Provide a double opt-in.
Authenticate your email with SPF, DKIM, DMARC, and BIMI.