There can be plenty of reasons for this email behavior:
- You are a spammer: you have your device or server infected with malware that is using your email address to send a huge amount of emails over internet. Remove the malware immediately and change your email password.
- Your email contains images: we know that you need that in your email footer, but some servers are very strict.
- You use links: some servers punish this behavior in case of repetitive action.
- You send to many: your email contains many emails on Cc.
And the list may continue. In most cases there is a combined recipe of the reasons above. Check if your emails (or web domain) are blacklisted and remove the blacklisting using the following tool: