Emails sent to hotmail/outlook addresses from our SMTP server disappear without an error

Hi,

recently, we have found out that we have quite many users who did not finish the registration process on our web. This means they did not click the email verification link.

It turned out that majority of these users have their email on outlook.com, hotmail.com, live.com ... these are Microsoft email services. So, we tried to create an account on hotmail and try ... the verification email never arrived.

Other services like gmail receive our mails without problems. OK, so why hotmail does not?

How do we send our emails? We use our own SMTP server, which is Windows Server 2012 IIS SMTP server. So, we've setup extensive logging and found out this to be a normal conversation between our SMTP server and hotmail SMTP server:

- - 220+SNT004-MC2F1.hotmail.com+Sending+unsolicited+commercial+or+bulk+e-mail+to+Microsoft's+computer+network+is+prohibited.+Other+restrictions+are+found+at+http://privacy.microsoft.com/en-us/anti-spam.mspx.+Fri,+13+Jun+2014+00:36:52+-0700+ 0 0 237 0 125 SMTP - - - -
EHLO - example.com 0 0 4 0 125 SMTP - - - -
- - 250-SNT004-MC2F1.hotmail.com+(3.20.0.129)+Hello+[130.193.10.74] 0 0 63 0 265 SMTP - - - -
MAIL - FROM:<>+SIZE=1489 0 0 4 0 265 SMTP - - - -
- - ....Sender+OK 0 0 49 0 390 SMTP - - - -
RCPT - TO:<> 0 0 4 0 390 SMTP - - - -
- - + 0 0 22 0 547 SMTP - - - -
BDAT - 1489+LAST 0 0 4 0 547 SMTP - - - -
- - 250++<>+Queued+mail+for+delivery 0 0 80 0 937 SMTP - - - -
QUIT - - 0 0 4 0 937 SMTP - - - -
- - 221+SNT004-MC2F1.hotmail.com+Service+closing+transmission+channel 0 0 65 0 1062 SMTP - - - -



If I read it correctly, this is perfectly normal (communication with gmail SMTP server looks almost exactly the same). The most important line is this:

250++<>+Queued+mail+for+delivery 0 0 80 0 937 SMTP

This tells me that hotmail SMTP server accepted our email and told us that it is queued.

So, why was it never delivered?

Note that this is not a problem of a spambox. If I send email to our hotmail address from different service, it works perfectly, some mails go to spambox, but are never lost. Only email from our SMTP server disappear. We have also tried to contact Microsoft support, but so far they think we are having troubles to receive emails to our hotmail inbox, which we are not ... they did not understand it at all.

Also note that our SMTP server IP address has valid PTR record that translates to mail.example.com, which is required by many big SMTP servers to prevent spam.

Any ideas?

 

SPF record, DKIM? Is this a new server? Does the IP itself have any previously bad reputation? If so only your service provider or the owner of your IP's can usually help by coordinating with them to clear the IP's from blacklists. The logging may help but once it leaves your network, its up in the air whether it goes all the way through or not.

 

SPF record / DKIM - do you have any links for this?

IP has no bad reputation, IP is no on any of 120 blacklists we checked.

I guess that if there was a problem with blacklists or spam or something then the server would not say "everything is OK, I will deliver". It would rather say "sorry, I will not deliver, you are not trusted." or something like that.

 

OK, I found http://www.openspf.org/SPF_Record_Syntax
and most importantly I found this wizard http://www.microsoft.com/mscorp/safety/content/technologies/senderid/wizard/default.aspx
which is easy to use generator of SPF records. I used it to generate new SPF record for our domain and will try it now.

 

Thanks ironcladservers, it seems that correction of our SPF record did the job.
Yet still I do not understand why hotmail SMTP server reported no error when discarding our emails