421 validating sender

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Microsoft's Smart Screen® technology is designed to provide anti-spam filtering innovations for as well as other Microsoft products like Exchange Server, Microsoft Office Outlook and Windows Live Mail.

We also leverage SPF , an email authentication technology protocol that helps address the problem of spoofing and phishing by verifying that the domain sending the email is authorized to do so.

When two email servers, or Mail Transfer Agents (MTAs), send and receive emails between each other, they communicate using a series of numeric codes.

These SMTP codes always take place in pairs, which means that both of the servers will transmit SMTP codes, until either the conversation is successful, or fails.

Date/Time Summary 2015-05-07 Delivered: sender → recipient 'You have a new notification' 2015-05-07 Will retry in 3600 seconds: sender → recipient 'You have a new notification' Server response: 421 421 Syntax error 2015-05-07 Will retry in 1800 seconds: sender → recipient 'You have a new notification' Server response: 421 421 Syntax error 2015-05-07 Will retry in 900 seconds: sender → recipient 'You have a new notification' Server response: 421 421 Syntax error 2015-05-07 Will retry in 600 seconds: sender → recipient 'You have a new notification' Server response: 421 421 Syntax error 2015-05-07 Accepted: sender → recipient 'You have a new notification' I am new to using Mailgun and I am building the emails RAW (headers and all). The error code of "4xx" indicates that this is a soft, temporary bounce.

Some of the messages go through fine, but I do have an awful lot that are getting retried. throttling emails sent from your domain which is also known as ESP throttling, 2) grey-listing of the IP in which the recipient server will first verify that the sending server is not sending spam before allowing delivery, or 3) a local server issue may have occurred such as the server being offline or misconfigured.

As a result domains require an SPF record for their mail systems if they want their emails to be accepted by those providers.

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New IPs that are added for domains that are authenticated under existing SPF records typically experience the added benefit of inheriting some of the domain's sending reputation.

I just checked mailgun's domain verification and one of the MX was unverified and then I checked it again and it verified. The error that you are seeing is due to the recipient's server either 1).

I'll have to wait until tomorrow to see if the retries start going through.

If you’ve taken a look at your spam box recently you’ll notice that a great many of those spam messages are attempting to use forged email addresses to trick you into opening them.

Forged Email addresses claiming everyone from Facebook, UPS, DHL, and the CIA have attempted to trick me into opening their emails.

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