550 5.4.6 Mail rate exceeded limit

550 5.4.6 Mail rate exceeded limit

Hi, one of our emails ( simone.zanatta@duetologistics.com) has been blocked several times on last two days and I'm seeing the following error when I attempt to send an email.

 550 5.4.6 Unusual sending activity detected. <a
href=https://www.zoho.com/mail/help/usage-policy.html target=_blank>Learn
more.</a> Use the link mail.zoho.com/UnblockMe to unblock

When I access the link mail.zoho.com/UnblockMe to unblock, the block type is Mail rate exceeded limit.

Why was detected unusual sending activity or mail rate exceeded limit? Or what is the Unusual sending activity detected?

Can support answer me?

Thanks in advance