Here are a few things to consider when troubleshooting why an email address is not able to be added to your contacts:

1. Are you entering the correct email address? Double-check you have the correct spelling and domain.
2. Have you checked the contact wasn’t already saved? Search for the contact first to verify it is not already there.
3. Do you have an account limitation on contacts? Check your account settings or service plan to verify how many contacts you can save.
4. Can you save contacts from other domains? Verify if you are able to save contacts from other domains than your own.

If all of the above have been verified and the email address still cannot be added to your contacts, then it’s likely the domain does not accept contact invitations from external email addresses.