My business email suddenly goes into spam with majority of people I email for the first time

The email address I use for business correspondence goes to the spam folder (Gmail, Hotmail, Yahoo) for the majority of people that I email for the first time.

To be clear, I am not involved in any kind of spam or cold emailing with this email. I think that I cold emailed maybe 2-3 people in the past 4 years.

It first started with Hotmail and Yahoo who are known to have a rigid policy when you email someone that is not in your address book. But in the past 3 months, my Gmail contacts are telling me that they have missed my email as it ended up in the spam.

So I checked my email with mail-tester.com and it reported that my email is OK. I got 3 warnings:

1. You do not have a DMARC record
2. Weight of the HTML version of your message: 5KB. Your message contains 12% of text.
3. Your message does not contain a List-Unsubscribe header

For 1, I think it is not necessary, but I will make it today. I thought that Hostgator is doing this. For 2, this is normal as my signature is 10-15 lines long (business signature) and I often send messages with just a few sentences. For 3, this is not any promo email so I do not need the unsubscribe.

Now, this is the environment that I use:

1. I use Gmail to sent emails for all my email addresses. Most of them are on Hostgator and I use Gmail to receive and send messages. I do not have Gsuite for all my emails.
2. I often receive an email to the general emails of my company, such as info@ or jobs@ and I reply with my real email [email protected]

PS. I did not notice that I have these issues on my Gsuite domains, but I do not use them as much as those from Hostgator. They are mainly for our internal communication, while the Hostgator ones are for external communication.

Any ideas what could be happening? Why am I being marked as spam?

DMARC exists for Gsuite domains, which work for you, but your domain doesn't work and it lacks it, so this is probably your problem.

Many email servers nowadays have become very strict about emails. This is not the first time that I see such posts as yours.