Order emails not sending to customer

Our order emails have stopped sending out to customers and we're trying to figure out why... the order places email goes to the store owner, but none of the customer order emails are sending. In the message queue there is no date in the sent column (they all have 3 sent attempts).
Here is the log error message:

Log level:   Error
Short message:   Error sending e-mail. Mailbox unavailable. The server response was: relay not permitted
Full message:   System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: relay not permitted at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, Boolean allowUnicode, SmtpFailedRecipientException& exception) at System.Net.Mail.SmtpClient.Send(MailMessage message) at Nop.Services.Messages.EmailSender.SendEmail(EmailAccount emailAccount, String subject, String body, String fromAddress, String fromName, String toAddress, String toName, String replyTo, String replyToName, IEnumerable`1 bcc, IEnumerable`1 cc, String attachmentFilePath, String attachmentFileName, Int32 attachedDownloadId) in c:\development\NopCommerce_Augustine_3_5\Libraries\Nop.Services\Messages\EmailSender.cs:line 130 at Nop.Services.Messages.QueuedMessagesSendTask.Execute() in c:\development\NopCommerce_Augustine_3_5\Libraries\Nop.Services\Messages\QueuedMessagesSendTask.cs:line 43

Any ideas where to look at fixing this one?

TIA :)

Oops - meant to say we're on 3.50

Is your default email account set up with a User name and Password?

Yep - the test email function works fine and the emails store owner notification emails work fine, just the customer notification ones don't seem to send...

I am having the same problem but it is not sending to the store or customer.
the test email works fine
I would appreciate any help with this.
Many thanks

shlomo wrote:
Which email is not sending to store [owner?] or customer? Did you check related message template (check here) is active or not?

Check the Message Queue.  Is the message still in the queue?
If so, then 'resend', and then check the System Log for any errors.

this is the message I get
System.AggregateException: One or more errors occurred. (A task was canceled.) ---> System.Threading.Tasks.TaskCanceledException: A task was canceled.
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at Nop.Services.Tasks.TaskThread.Run() in C:\andrei\nopcommerce\sources\src\Libraries\Nop.Services\Tasks\TaskThread.cs:line 71
---> (Inner Exception #0) System.Threading.Tasks.TaskCanceledException: A task was canceled.<---

I cleared the log and it started working all sees fine
thanks for your help.