We're sending emails from a site using the mojoPortal email mechanism mojoPortal.Net.Email, but sometimes hitting limits in the Office 365 email relay. Entries like this in the log
2020-08-27 14:52:32,736 ERROR (null) - (null) - (null) - mojoPortal.Net.Email - error sending email to email@example.com from firstname.lastname@example.org, will retry
System.Net.Mail.SmtpException: Syntax error, command unrecognized. The server response was: 4.3.2 STOREDRV.ClientSubmit; sender thread limit exceeded [Hostname=DB6PR0701MB2248.eurprd07.prod.outlook.com]
at System.Net.Mail.DataStopCommand.CheckResponse(SmtpStatusCode statusCode, String serverResponse)
at System.Net.Mail.DataStopCommand.Send(SmtpConnection conn)
at System.Net.Mail.SmtpClient.Send(MailMessage message)
at mojoPortal.Net.Email.Send(SmtpSettings smtpSettings, MailMessage message)
We think we're simply exceeding the number of concurrent sends and perhaps also the number of messages per minute that are supported by O365, as per https://docs.microsoft.com/en-us/exchange/troubleshoot/send-emails/smtp-submission-improvements
I've seen that the mojoPortal Newsletter and Forums both have settings to throttle the email sending.
<add key="NewsletterMaxToSendPerMinute" value="0" />
<add key="Forum:NotificationMaxToSendPerMinute" value="0" />
Is there similar for global emailing? Any other suggestions? (Using a different email provider is not a solution available in the immediate situation).