What are the reasons emails sometimes fail to reach certain users or get bounced back?

There are several reasons why emails get added to the bounce list, which prevents them from being delivered to the intended recipients. Some of the common reasons are listed below:

• Suppression Lists: User's email address might be in a suppression list. Suppression lists are used to prevent sending emails to addresses that have previously bounced, unsubscribed, or marked emails as spam. If user's email address is in such a list, any emails sent to the user will be automatically blocked to protect the sender's reputation and improve deliverability rates source.
  
• Email Address Issues: Sometimes, emails bounce due to issues with the recipient's email address. This could be due to a typing mistake, the email address no longer being active, or the domain being invalid.
  
• Sender Reputation: If the emails are frequently marked as spam, their emails might be blocked or filtered out by email providers, leading to bounces.
  
• Technical Issues: There could be technical issues on the sender's side, such as misconfigured email servers or authentication problems, which can cause emails to bounce.
  
• Email Filters: User email provider might have filters in place that block certain types of emails, especially if they are perceived as spam.