Unable to send message reason 550 5.4.6 unusual sending activity detected.

Unable to send message reason 550 5.4.6 unusual sending activity detected.

Hello,

I am writing in order to ask for help regarding the issue l am facing with. I am trying to send some emails to companies to promote my services, but after 15-20 sent emails I was unable to send emails anymore with this message pops up: 
unable to send message reason 550 5.4.6 unusual sending activity detected.
What can I do to avoid this message and to continue to sending emails to potential clients? Can you advise me is there an option like that on Zoho? It's pretty urgent so I hope I'll heard from you soon!

Thank you,
Ivan

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