Source: Sendinblue (email server)
As an email sender, your reputation is the most important thing that determines whether your messages will reach the inbox. Your reputation is based on three primary factors:
1. History with your contacts
Do you have a positive history with the contacts you are sending to? If they have a history of opening/clicking emails from your domain (firstname.lastname@example.org), future emails are likely to be delivered as well. This is completely independent of the IP address or email marketing provider you are using. If contacts did not recently subscribe to receive the emails you are sending, their behavior will be recognized quickly by ISPs and associated negatively with your domain.
2. Domain reputation
Even if you are sending to new contacts for the first time, ISPs will deliver your emails based on your reputation with them. For example, if other Gmail contacts ignored your emails, marked them as spam or unsubscribed in mass, there is a good chance that Gmail will recognize your domain as sending spam and put you in the spam box going forward. The opposite is also true - if your reputation with an ISP is positive, this will help you reach the inbox when contacting new people for the first time.
3. Other questions:
Which IP address is sending your email?
Similar to domains, IPs also carry a reputation. However the domain is the most important when sending from a neutral or new dedicated IP.
What is your email’s content?
ISPs monitor content for indications of spam.
Are you reported in a blacklist?
There are multiple third-party agencies that maintain blacklists for the major ISPs. If you are blacklisted by one of these agencies, you usually receive an email message explaining why and providing steps to become un-blacklisted. Ignoring these will permanently damage your deliverability.
Why did a participant not receive an email with the status "sent"?
There are different levels of spam filters. It is possible that an email has arrived at the recipient's server but has been blocked by another filter. These last filters do not report this back to the sender server. Therefore, the status may be sent even though the recipient did not receive the email.
Further information on the SPAM topic