How to send an email from office 365 using an alias

Realized we can't send out from an alias email address using Office 365 / Online Exchange. Well, we can choose to do so but external recipients see our primary SMTP address. I made the alias primary SMTP and waited a bit but same thing. I then removed the alias and created a Shared Mailbox with the alias as the primary SMTP and waited a bit but surprisingly external recipients still saw the primary SMTP from our "main" account.

For example, I removed smtp:me@aliasdomain.com from my user / email account and created a shared mailbox with me@aliasdomain.com as the primary SMTP email address. I received a UPN error when trying this so I simply changed the display name on the shared mailbox to "Me Alias Domain" from just "Me" (which matched my primary account). That worked to create the shared mailbox and the me@aliasdomain email address could be typed into Outlook and used to send an email. Internal recipients saw the alias domain but external recipients saw the primary domain email address.

Any ideas how to send out from Office 365 using an alias without a third-party tool are much appreciated.
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king daddyAsked:
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David Johnson, CD, MVPRetiredCommented:
Don't even think of using this.. Spammers would love it.
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king daddyAuthor Commented:
We need to prove we "own" a domain. Registrar records were not good enough (the person needing to do this requested the records from us - IT - which we provided and he sent along to the other org but they wanted an email from the domain we are claiming to own - which we obviously do). May sound crazy but he forwarded the emails to us that he received from them stating all of this.

Thanks for the reply
David Johnson, CD, MVPRetiredCommented:
Add a mailbox just for this and delete it after a month.
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Vasil Michev (MVP)Commented:
There are add-ins that allow you to send as alias easily, but since you dont want to use them, the approach you described above can be used. Once you create the new recipient with the alias in question, you need to also grant Send As permissions (not send on behalf) and make sure to send AS the alias (select the address directly from the GAL, dont use any stored From or auto-complete entries). Or you can make it simpler by creating a regular user mailbox for said alias, then login directly with the credentials for that mailbox and send the message.

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king daddyAuthor Commented:
Thanks David, I guess I am going to do that.

Thanks Vasil. I created a shared mailbox and used the alias as the primary SMTP but when the user sends the email out it still shows his primary SMTP from his primary domain and not the alias domain. I wonder if that is because the shared mailbox is opened in his primary domain's outlook. Weird thing is we have several shared mailboxes with lots of people having full access as well as send as, and the shared mailbox is open under Outlook, and they do not have this issue of the primary domain / email address showing externally.

Could it be an issue with the Display name in EAC and Office 365 being the same for both the shared mailbox (alias email address) and his primary mailbox (primary domain email address)?
king daddyAuthor Commented:
OK I just tried again and, taking Vasil's advance to choose from the GAL instead of autocomplete, it worked.

Thanks all.
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