Randomly getting error sending emails

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At the end of the approval process I'm sending email with some details and all atachments to specific mailboxes. But from time to time I'm getting error "ErrorAccessDenied
Message: Access is denied. Check credentials and try again". Credentials are ok - it is sending 100 emails and giving this error on 10 another cases. Might be connected that Webcon app is sending 100 emails in like 2-3 minutes and Microsoft Graph has some limitations?