Mail not arriving to Office 365

Question

Hello, I use the API to send email with MailForWindowsStore.dll to Office 365. the problem is that nearly 50% of the email does not arrive. After opening a case with microsoft, it appears that the email does not arrive to O365 they are simply lost ..... we enable logs on the client dll, but no error appears. every thing seems fine. Have you ever had a similar problem?

Answer 1

Are you examining the return value of Smtp.SendMessage method?

Have you ever had a similar problem?

No, never.

we enable logs on the client dll, but no error appear

In such case it seems that your SMTP server accepted the message and it is now responsible for delivering the message or generating a failure report.

Why do you think the problem is with Outlook 365? Maybe it's your SMTP server, that is swallowing the messages.

Are you sure you're not treated as a spammer?

Comments

the smpt server is smtp.office365.com. Microsoft says that they have no trace from an incoming message.
We have no problem when we use a power shell cmd like

Send-MailMessage -From XXX@YYY.com -To toto@yahoo.com,XXX@YYY.com -Subject "test toto $date" -Body "Test SMTP Relay Service" -SmtpServer smtp.office365.com -UseSsl -Port 587 -Credential $Msolcred

maibye we are treated as spammer ...

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How does the Mail.dll log look like for such not-delivered message?

Can you try disabling pipelinig and chunking just to be sure:
smtp.Configuration.EnableChunking = false;
smtp.Configuration.EnablePipelining = false;

Is there anything different about those messages? E.g. Only messages to many users are not delivered?