Source from Sendinblue, an email server:
Your email reputation is the most important element in ensuring that your messages arrive in your contacts' inboxes.
Your reputation is essentially based on 3 factors:
1. history of your relationship with your contacts.
Is the history of the relationship with your contacts positive? If your contacts have previously opened and clicked on emails received from your domain (firstname.lastname@example.org), future emails are guaranteed to arrive in their inbox. This history is independent of your IP address and the emailing solution you use.
If contacts have not recently subscribed to your newsletters, providers will quickly take note of their behaviour in relation to your emails (unsubscribing or moving them to the spam folder) and negatively associate it with your domain.
2. Domain reputation.
Campaigns sent for the first time to newly registered contacts are also delivered by providers according to the general reputation of your domain. For example, if Gmail contacts have previously ignored your emails, moved them to the spam folder or unsubscribed, Gmail may consider your domain to be spam and automatically move your messages to the spam folder for all Gmail recipients.
If your reputation with providers is positive, your messages will systematically arrive in the inbox, both to current and new contacts.
3. Further Questions
Which IP address will be used to send my email campaigns?
Like domains, IP addresses have a reputation with providers. Remember that the domain is the most important element for your messages to arrive in the inbox - this applies to shared IP addresses as well as dedicated IP addresses.
How important is the content of my emails for delivery to the inbox?
Providers check the content of your messages for evidence of spam.
Am I blacklisted?
There are several third-party organisations that provide blacklists of the major providers. If you have been blacklisted by one of these solutions, you will usually receive an email explaining why and the steps required to be removed from the blacklist. Ignoring these steps may have a negative impact on your deliverability.
Why didn't a subscriber receive an email with the status "sent"?
There are different levels of spam filters. It may be that an email has arrived at the server of the recipient, but was then blocked by another filter. These last filters do not report this back to the sender server. Therefore, the status can be sent, although the recipient has not received the email.