Microsoft 365 and Outlook recipients apply their own filtering and reputation checks. Using a Microsoft mailbox does not guarantee that every message reaches the inbox.
Improve the basics
Use verified, relevant contacts and accurate sender information. Keep the message concise, avoid unnecessary tracking, and respect declines and opt-outs.
For sending.ac mailboxes, leave managed authentication and nameservers in place and follow the sending settings shown in your account. Allow at least 3 weeks of preparation for new mailboxes before campaigns.
If performance changes
Compare similar audiences and record the domains, dates, bounce messages or placement tests involved. Pause the affected campaign if failures increase and ask support to review the evidence.
Do not diagnose delivery solely from a domain's age, provider label or IP address. A clean technical setup and a relevant campaign need to be considered together.
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