Using New Outlook on Windows for personal email, calendar, and contact management
I had the same problem with a fairly new Outlook.com address, so I can explain what is happening. Outlook.com applies daily sending limits, and new accounts start on a much lower temporary quota until they build up credibility in the system. That is why your older private account is fine while the new one gets blocked or asked to verify.
A few things that helped me:
- Keep the batches small. Around 20 at a time is sensible, and spreading the sends through the day helps too.
- Put the whole list in Bcc instead of To or Cc. It protects everyone's addresses, avoids reply-all chaos, and a long visible recipient list is exactly what spam filters flag.
- Add your regular readers to your Outlook.com Contacts, and ask a few of them to add your new address to their safe senders list. Microsoft's own guidance says limits ease as you communicate regularly with contacts.
- Finish verifying the account: phone number, alternate email address, and the Microsoft Authenticator app if you use it. Fully verified new accounts get trusted faster.
On your Microsoft 365 question: the higher sending limits apply to the Microsoft account that holds the subscription, and they still vary with usage history. If the new address is on a different free account, its own limits apply, which is probably why you still see the upgrade prompt. The official figures are in Microsoft's article "Sending limits in Outlook.com": https://support.microsoft.com/en-us/outlook/sending-limits-in-outlook-com
Keep using the address normally for a few weeks and the quota rises on its own.