How do I know if emails are being rejected?

The best way is to look for bounce messages and delivery failure reports.

Signs emails are being rejected

• Hard bounce: immediate rejection because the address or domain is invalid, blocked, or doesn’t exist.
• Soft bounce: temporary issue, like mailbox full, server down, or message too large.
• SMTP error codes in your mail logs, such as:
  • 550 / 551 / 553 often mean permanent rejection
  • 4xx codes often mean temporary failures
• Non-delivery reports (NDRs) or “Mail Delivery Subsystem” emails sent back to the sender.
• If you use an email service provider, check their delivery dashboard for:
  • bounce rate
  • rejected count
  • deferred messages
  • spam complaints

If you manage your own mail server

Check:

• SMTP server logs
• queue logs
• postmaster reports
• spam/filter logs

If you’re sending from an app or website

Make sure you’re capturing:

• the SMTP response from the server
• webhook events from your email provider
• bounce notifications

Common reasons for rejection

• invalid recipient address
• recipient server blocks your domain/IP
• poor sender reputation
• missing SPF/DKIM/DMARC
• message content flagged as spam
• policy restrictions on recipient side

If you want, I can also show you how to check this in Gmail, Microsoft 365, or a mail server like Postfix/Exim.