Microsoft weights sender reputation via SNDS and JMRP, and rejects unauthenticated bulk mail to consumer Outlook.com addresses.
What email subject lines are#
The subject line and preheader are the two text fields shown in the inbox list before a message is opened. They determine whether a recipient opens, ignores, or reports the message.
Why it matters#
Subject lines are the highest-leverage copy you write. They also affect complaint rates, which feed reputation.
Setting up email subject lines for Outlook and Microsoft 365#
- Keep subject lines under 50 characters so they survive mobile truncation.
- Write the preheader as a continuation, not a repeat of the subject.
- A/B test one variable at a time with a statistically meaningful sample.
- Avoid deceptive 'Re:' or 'Fwd:' prefixes, which violate CAN-SPAM and raise complaints.
How Outlook and Microsoft 365 reports results#
Open a delivered test message in Outlook and Microsoft 365 and view the original or headers. Look for Authentication-Results and any provider-specific verdict headers. Use the provider's sender dashboard for aggregate reputation.
Common mistakes#
- Clickbait that produces opens but drives complaints and unsubscribes.
- All-caps and excessive punctuation, which pattern-match to spam.
Frequently asked questions#
Do emojis in subject lines hurt deliverability?
Not directly. They can hurt engagement with B2B audiences and render inconsistently across clients.
Does Outlook and Microsoft 365 require email subject lines?
Microsoft weights sender reputation via SNDS and JMRP, and rejects unauthenticated bulk mail to consumer Outlook.com addresses. Treat email subject lines as required for any meaningful volume.