It often happens that you wait for an email, and it is delayed. Later you find out that it has arrived but in fact it has entered SPAM. You mark the email as NOT SPAM and move it to INBOX. However, the issue is not fully resolved. GMAIL may reject messages repeatedly, or worse, block them from the first connection to their servers. When this happens, you receive a message with a rejection report that looks something like this:
Analyzing this report is quite difficult because GMail does not provide a clear explanation of the reason for the rejection and an exact procedure for fixing the problem. GMail often encounters these situations and the email sent is considered suspicious to their servers, automatically raising an alarm. The domain's reputation as a sender will suffer, including G-managed domains, and the sent messages will begin to be rejected. The domain's reputation is quite difficult to restore, because when GMail starts rejecting messages, there is no longer an option for recipients to mark it as NOT SPAM and correct GMail's behavior.
The first step in trying to fix the problem is to get more detailed reports by registering the domain in Postmaster Tools where you can see the delivery issues. Messages are filtered based on content, title, signature, and keywords written in the email. As a result, emails will need to be adapted to GMail requirements, but that's not all. GMail's approach to mail traffic is based on software that learns and analyzes trends and patterns in real time, correcting its own mistakes. Filters will be folded and adapted for each GMail user according to their behavior and will be adapted to their preferences. If most recipients do not open or click on your emails at all, they will give GMail the impression that these messages are not important. Moreover, if someone marks the message as SPAM, GMail will categorize the domain as unsolicited. This combination of factors will reduce the domain's reputation until it is completely blocked by the GMail server. Consequently, the content of the e-mail must be taken into account and it must always be viewed by the recipient.
To sum it all up, free email service providers try to keep their email traffic as legitimate as possible by preventing spam, spam emails, and other unwanted or unsolicited messages.