Why are email notifications not being sent when creating events via Nylas? [Microsoft Grants]

Note: This does not apply if you are missing Nylas Scheduler notifications. Please contact support if that is the case. - Contact Support 

Error:

"Your message wasn't delivered because the recipient's email provider rejected it."

Remote Server returned '550 5.7.501 Service unavailable. Spam abuse detected from IP range.

When creating calendar events through Nylas which uses Microsoft Graph API, participants may not receive email notifications due to Microsoft's email delivery restrictions. This typically occurs when:

1. The sending IP address has a low reputation score with Microsoft
2. The tenant is new or using a trial license
3. The remote server returns error '550 5.7.501 Service unavailable'

This is a Microsoft-specific limitation. The following solutions typically resolve this issue for other Microsoft users:

1. Contact Microsoft Support to request a sending exclusion for your tenant
2. The exclusion typically lasts 60 days and allows the use of blocked IPs while using a new/trial license
3. Provide Microsoft Support with the specific error messages and timestamps of failed delivery attempts

This issue apparently affects API-created events - events created directly through the Microsoft interface will continue to send notifications normally.

Here are some Microsoft Forums sources:

- https://answers.microsoft.com/en-us/msoffice/forum/all/event-notification-mail-not-going-when-created/fc6037a8-cb36-4646-a3b2-eca2f4fdae17 

- https://learn.microsoft.com/en-us/answers/questions/1084092/email-notifications-not-being-sent-while-creating 

- https://learn.microsoft.com/en-us/answers/questions/819036/not-able-to-send-email

- https://learn.microsoft.com/en-gb/answers/questions/1693970/how-do-i-fix-email-bounce-backs-when-using-the-gra 

- https://learn.microsoft.com/en-us/answers/questions/611558/create-event-api-not-sending-attendees-email-or-ad

Related issue regarding SENT email message getting bounced:
- https://support.nylas.com/hc/en-us/articles/5162469060765-Emails-sent-via-the-Nylas-API-go-into-spam-or-return-a-bounced-message