Spam folder is the storage in your email account for unwanted emails or for emails not succeeding to reach the inbox and being flagged as spam by email service providers (ESPs). It is also known as "Bulk Folder" or "Junk Folder".
These filtering systems are established to help users decrease the amount of received unnecessary and promotional emails.
When you send a message, email service providers (ESPs) look at your reputation of email address and the content of the email message. If there is any aspects looking suspicious, ESPs automatically block your email to reach the targeted inbox. Receivers are also able to flag an email as containing spam.
Sign in your Gmail account
Under the "Categories" section, click on "More".
There may be email spam filter issue
Sender's email may has low sender score
The IP adress may not be reputable.
Domain authentication requirements may be neglected.
The domain may not be reputable.
The sender may not obtain customers' permission.
Email sending frequency of sender may not be set.
The sender may has low email open rates.
Email list of sender could not be fresh.
There may no unsubscribe link.
Sender information could be incorrect.
The content of the message may include spelling mistakes and grammatical errors.