Help for Postmaster sending issues
Mailer Daemon or postmaster are one or the same thing. This generally happen when the Mail send get Returned.
This generally happens when you send an Email to a person whose Email no more exists or you are sending to someone but the Email Address is not spelled correctly they you might get to see the Postmaster.
Now generally we get two types of Error Codes
1) 4xx SMTP error codes.
This error indicates that there is a temporary problem, because of which the mail send is undelivered. The reason for this Hotmail Mail Postmaster error could be, unusual traffic pattern from your sending server IP Address. Or the Email that you are trying to send may looks like a spam mail or the content in the mail comes under spam, or may be your mail server is creating the problems for Hotmail Mail users.
2) 5xx SMTP error codes.
This error indicates that there is a Permanent problem, because of which the mails send has been returned. The reason for this Hotmail Postmaster Error could be invalid Email Address, or the Email Address no more exists, SMTP connection has been rejected, or there could be many reasons for these Problems.
In case of 4XX SMTP Errors you have to wait for a while like 24 Hours, and after that you might not get the same error while sending.