Random System.Net.Mail.SmtpFailedRecipientsException exception - relay account office365 - smtp

Question

We have an application that runs on a Windows Server 2008 machine. It sends emails out using an office365 smtp relay account. However, all the emails are not sent successfully. We randomly get these two exceptions on emails being sent out on the smtp.Send call:
System.Net.Mail.SmtpFailedRecipientsException: Unable to send to all recipients. ---> System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: 5.7.64 TenantAttribution; Relay Access Denied
System.Net.Mail.SmtpFailedRecipientException: Insufficient system storage. The server response was: 4.5.3 Too many recipients
Thus far, we haven't been able to figure out why this is happening. Any ideas are appreciated.
The email code uses System.Net.Mail namespace - .Net framework 4.0.
We pass in the username and password for the NetworkCredential.
public void Send(string from, string[] to, string[] cc, string[] bcc, string subject, string body, string[] attachmentArr, Boolean isBodyHtml, string smtpServerName, int port = 25, bool enableSsl = true, string userName = null, string password = null, string domain = null, int timeoutMilliSec = 100000)
{
MailMessage objEmail = new MailMessage();
try
{
foreach (string toItem in to)
{
objEmail.To.Add(toItem);
}
if (cc != null)
{
foreach (string toItem in cc)
{
objEmail.CC.Add(toItem);
}
}
if (bcc != null)
{
foreach (string toItem in bcc)
{
objEmail.Bcc.Add(toItem);
}
}
objEmail.From = new MailAddress(from);
objEmail.Subject = subject;
objEmail.Body = body;
objEmail.IsBodyHtml = isBodyHtml;
objEmail.Priority = MailPriority.High;
if (attachmentArr != null)
{
foreach (String s1 in attachmentArr)
{
objEmail.Attachments.Add(new Attachment(s1));
}
}
using (SmtpClient smtp = new SmtpClient(smtpServerName))
{
if (string.IsNullOrEmpty(userName) == false && string.IsNullOrEmpty(password) == false)
{
NetworkCredential credential = (string.IsNullOrEmpty(domain)) ? new NetworkCredential(userName, password) : new NetworkCredential(userName, password, domain);
smtp.Credentials = credential;
}
smtp.Timeout = timeoutMilliSec;
smtp.Port = port;
smtp.EnableSsl = enableSsl;
smtp.Send(objEmail);
}
}
catch (Exception ex)
{
throw ex;
}
finally
{
if (attachmentArr != null && objEmail.Attachments != null)
{
foreach (Attachment a1 in objEmail.Attachments)
{
a1.Dispose();
}
}
}
}

Answer 1

We finally found what was going on - Email throttling.
Office365 has a Throttling limit of 30 messages per minute for SMTP client submission https://technet.microsoft.com/en-us/library/dn554323%28v=exchg.150%29.aspx#summary
The solution was to send less than 30 messages per minute. I think it is affected by other messages (sent by Outlook) showing up to be sent by the server too. We pushed ours down to almost a five second delay between messages. We have not seen the error reoccur since.