In this post, I'd like to talk about what issues I had in the past regarding emailing.
First, let's talk about your professional email address (business related). When you sign up to a hosting company, you have the option to create email addresses associated with your domain - once you start using their DNS. For example, [email protected] or [email protected]
There is a high chance the IP address you get is a shared IP (used by a lot of other users). Thus, its reputation is damaged, especially if it was used by a lot of spammers. As a result of this, even with a simple "hello world" message, it's impossible to hit Gmail inbox. So those emails will land in the spam folder. You don't want to send messages to spam, right?
So I suggest you to always test it by sending some messages to your own Gmail addresses (or some other providers). Once you realize they hit the spam folder, you need to investigate further.
Open your email in Gmail and click the three dots, then click show original. (By the way, here you can see the different records - SPF, DKIM whether they passed or failed. If they failed, then you have some additional configuration problems.) On the top, you can see the sending IP address - take note of it.
Now it's time to check the reputation of that IP address. There are two tools I recommend:
Mxtoolbox and Ultratools
Go there, paste the IP address and they will show you whether they blacklisted or don't. (Please note not all blacklist databases are relevant) If you find some, contact your hosting provider to initiate blacklist removal. Be patient, it takes time to resolve and sometimes the reason for spam is a different thing.
How to solve this instantly and what I am doing, too
The best thing to fix this is to connect your business domain to GSuite. I highly recommend you to do this even if your professional emails don't go to the spam folder because it's so much more reliable and you get a lot of additional benefits. Also, GSuite with its excellent reputation ensures your emails hit inbox all the time. However, its a paid service - $5 / month.
A similar case can happen when you sign up to an email marketing software that has a poor reputation. In that case, no matter what you do, how you change the content, etc., you won't be able to avoid spam folder. Nowadays, server reputation is the ultimate factor in deliverability. So before integrating it into your production site, always send test emails to a few of your Gmail addresses (or other providers).
How to check it
Use the methods above to find out the sending IP address, then go to senderscore.org to get to know the sender reputation. Sender Score is a number between 0 and 100 that identifies your sender reputation and shows you how mailbox providers view your IP address. A score below 80 is weak.
Another tool you can use is Talosintelligence. It will be shown as poor, neutral or good.
If you find out that the email marketing tool you're using has a bad reputation, I suggest you cancel your account and start using another one. Reputation is not something that can be changed overnight. (Or you might contact them first to let them know. An email marketing software usually has more servers divided into IP pools. Some pools might have a better reputation, and some might have poorer. On your request, your email marketing company might be able to change it for you.)
The most popular email marketing companies have a dedicated team who are constantly monitoring their server's reputation and acting accordingly before something bad happens. So always think at least twice which email marketing program you want to sign up to. And test and test.
These are things I experienced in the past. That's why I share them with you. Have you ever met similar issues?
I hope this info will be useful to you