Hello,

We use office 365 for our company emails and we're now setting up company b and wish to do the same.

I believe it is possible to use 1 office 365 account and add another domain https://support.office.microsoft.com/en-us/article/Add-additional-domains-to-Office-365-86f439e3-6f4b-4e84-96a9-19ef46bb5f10?CorrelationId=9dcf3008-7d5b-4d2b-ac52-03505e721d8b&ui=en-US&rs=en-US&ad=US

The question I have is it possible that user@companyA.com can open an additional mailbox from user@companyB.com and then send as that user.  I still need to keep the emails from both companies separate.

Thank you

Hi,

According to description, the issue is related to multiple domain mailbox in Exchange Online. Since this forum focuses on Office 2013 and Office 365 ProPlus which are the version of Office that comes with many Office 365 plans. Therefore, I suggest we can post the issue in Microsoft Exchange Online (Office 365) forum for more professional suggestions:

http://social.technet.microsoft.com/Forums/en-US/home?forum=onlineservicesexchange

Personal opinion, User@domainA.com can set user@domainB.com as a shared mailboxes and assign Full Access permission and Send As permission to User@domainA.com. If user@domainA.com has full access and Send As permission to user@domainB.com, he can add user@domainB.com as additional mailbox and send as this mailbox in OWA and Outlook. About these permission in Exchange Online, please refer to:

https://technet.microsoft.com/en-us/library/jj919240(v=exchg.150).aspx

Additionally, here are some similar thread about your issue:

http://community.office365.com/en-us/f/148/t/226425.aspx

http://community.office365.com/en-us/f/148/t/51020.aspx

Regards,