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This usually happens when the email service provider notices an unusual email traffic, let's say your client's email is receiving multiple emails from email@example.com, that will trigger his provider that that the source is sending spams. What he can do is that he can add the "sender email" in his/her contact list or whitelist us instead, depending on the options the email client has.
You can also try resolving this using a custom Sender Email or SMTP:
I hope that helps.