confusion creating new mail users in hybrid environment

we are running 365 hybrid
the on premise version is 2013
during migration we always created new users on prem then migrated them
We now have nearly all users migrated
obviously it is more time consuming creating new users on prem then migrating them
so we tried using the new office 365 mailbox option from the on prem console
this created the account ok but seems to be issues when sending / receiving external mail
internal is fine
the same applied when we created a shared mailbox on 365
some issue to do with the guid i think
must i always create the user on prem then migrate when in hybrid mode?

this was the info i found online that related to the shared mailbox

on-premises Exchange environment has no object to reference for the shared mailbox. Therefore, all queries for that SMTP address fail.
Create a remote mailbox in the on-premises environment, and then move the mailbox to Exchange Online. To do this, follow these steps:
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Vasil Michev (MVP)Commented:
No, you can simply create a remote mailbox. Just make sure you provision the correct routing address, and that mail users are in the scope of your email address policies (or simply configure the primary SMTP manually). Other than that, you have to be more specific about the external mail issue.

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