The following is a summary of best practices for sending mail to Microsoft email users, including bulk sender guidelines, deliverability factors and sending from new IP addresses. Implementing these best practices may help improve your sending reputation and inbox placement, but it is not guaranteed.

Best practices

• Encourage recipients to add your sending address to their safe senders list. Email addresses in the safe sender's list bypasses the SmartScreen filter and is delivered to the inbox.
• Sign up with Microsoft’s Junk Mail Reporting Program (JMRP) and suppress all complainers from receiving future emails.
• Sign up with Microsoft’s Smart Network Data Services (SNDS) to help troubleshoot delivery and sending reputation problems.
• Comply with all technical standards for the transmission of email, including Request for Comment RFC 2821 and RFC 2822.
• After receiving a Simple Mail Transfer Protocol (SMTP) error response code between 500 and 599, do not attempt to resend the message to that recipient.
• Do not open more than 500 simultaneous connections to Microsoft email servers. Send no more than 50 messages per connection. Microsoft may adjust your connection and throughput based on your sending reputation. Monitor your SMTP error codes for deferral messages and adjust accordingly.
• If you start to see 4xx deferral messages from Microsoft, Microsoft recommends that you stop all sending from that IP address for at least one hour and resume sending at a slower rate. If the deferral messages continue after you resume sending, stop sending from that IP for at least 24 hours.
  • Refer to Microsoft's SMTP error codes for additional information related to the deferral messages.
  • Investigate and fix the cause of the deferral messages. You especially want to be sure they are not caused by unauthorized access to your IP address.
  • If you continue to send at a high rate when seeing the deferral messages, it can negatively impact your sending reputation, which may lead to a decrease in inbox placement.
• Do not send email through an open relay or open proxy server. Make sure all email sent from your servers is authorized.
• Do not send email from a dynamic IP address.
• IP addresses must have valid reverse Domain Name System (PTR) record.
• Always include valid, reputable URLs in your email content. Do not link to known phishing websites. Make sure it is clear where the recipient will be taken and whether the destination is a valid website.
• Use the standard URL format. Avoid using IP addresses in the URL.
• Authenticate all email using Sender Policy framework (SPF), DomainKeys Identified Mail (DKIM), and Domain-based Message Authentication, Reporting, and Conformance (DMARC).
• Clearly brand email with a recognizable friendly-from address so that recipients are able to easily identify you.
• Suppress unengaged subscribers who do not want to receive emails from you.
• Use the list-unsubscribe header with the mailto: option in all email streams.
  • If you do not use the mailto: option, Microsoft will give your subscribers an option to block your future email messages.
  • Using the mailto:option will give your subscriber an option to unsubscribe from your email instead of blocking you, and future messages will be placed in the spam folder. Be sure to immediately honor all unsubscribe requests once received.
• Include a prominent unsubscribe link in all emails and immediately honor all unsubscribe requests. This should be done in addition to using the list-unsubscribe header.
• Warm up new IP addresses and make sure to update your Junk Email Reporting Program (JMRP) account to receive complaint feedback. New IP addresses using the same Return-path domain inherits the domain reputation, which can help with the warm up process if you have a good domain reputation.
• Send messages no larger than 25 MB.
• Do not use scripting languages in email design such as ActiveX or JavaScript.
• To ensure that messages are not marked as being from an unknown sender, join Return Path's Certification program. Return Path’s proprietary allowlist provides Microsoft with a list of responsible senders with excellent performance metrics in categories such as complaints and spam traps.
• Don’t attempt to verify the recipients email address without sending a message (namespace mining). Microsoft perceives this behavior as a spammer trying to harvest email addresses or a potential dictionary attack.
• Use opt-in consent methods. Do not purchase lists or engage in list harvesting techniques from websites and social networks.
• Set proper expectations about the content and frequency of your email. If possible, use a preference center.
• Onboard new subscribers into your marketing email stream.
  • New subscribers are people that have recently opted-in to receive your email and have not received previous marketing email from you.
  • Send a welcome email to all new subscribers. It is best to trigger a welcome email to a new subscriber once they have opted in. However, if you send a bulk welcome message to multiple new subscribers at the same time, split up the send in small batches and send over a period of time instead of all at once.
  • If you don't send a welcome message and just add new subscribers to your regular day-to-day marketing email stream, ensure that new Microsoft subscribers do not exceed 5% of the total Microsoft subscribers on any particular send.
    
    • For example, if you are sending to 100000 Microsoft subscribers for a single campaign, make sure that new Microsoft subscribers do not exceed 5000. 
    • Split up your campaigns if your new Microsoft subscribers exceed 5%.
• Avoid excessively urgent calls to action, such as Must sign up today!, Log in right now!, and Update immediately!. These are commonly used by spammers and could contribute to deliverability problems with Microsoft. Using urgency in your calls to action such as Buy now, Download our app, Start saving, Get your coupon, Learn more, and Shop now is a best practice, so try testing variations of your call-to-action to find out what resonates with your subscribers.