SharePoint Online external access invitations not being sent

Question

I have external access enabled on a site collection in my tenancy, and was previously able to invite external users to the site successfully, but now when I attempt to add external users nothing happens.

I don't receive any error messages, so it appears that the invitation has sent, only the user never receives the email. When I go to the Access requests and invitations page they are not in the External User Invitations list.

I set up a dummy Outlook account and was able to grant access to that user, but other domains aren't working.

However, I have added users from other domains in the past, so I can't understand why it worked for some users and not for others?

Answer 1

In SharePoint Online external access invitation will not get sent if invite message length is more than 255 character.

If you provide invite message with more than 250 character and invite. It closes window like invite has been sent but it will not send. Apparently , Microsoft has not provided any validation for it.