Re: Configuring two domains into 1 O365 Exchange account?

Hi Greengiant,

I’ve checked your domain name, it seems all the DNS are correct.
According to the NDR you provided, generally the error code “#< #5.1.1 smtp;550 5.1.1 RESOLVER.ADR.RecipNotFound; not found> #SMTP#”  is caused by the recipient doesn’t exists.

I suggest you double check if the user can be found in the user page.
Please refer to the following steps to check the user:
1) Sign in Office 365 portal
2) On Admin page, click Users under Management
3) You will enter Users page and check if the recipient is listed in the user list.

Thanks,
Vivian Wang