[Exchange Online] Email forwarding will work without license?

hi Takumi

Email forwarding requires that the account has a license. If you're setting up email forwarding because the user has left your organization, another option is to convert their mailbox to a shared mailbox. This way several people can access it. However, a shared mailbox cannot exceed 50GB.

You must be an Exchange administrator or Global administrator in Microsoft 365 to do these steps. For more information, see the topic About admin roles.

In the admin center, go to the Users > Active users page.

Select the name of the user whose email you want to forward, then open the properties page.

On the Mail tab, select Manage email forwarding.

On the email forwarding page, select Forward all emails sent to this mailbox, enter the forwarding address, and choose whether you want to keep a copy of forwarded emails. If you don't see this option, make sure a license is assigned to the user account. Select Save changes.

To forward to multiple email addresses, you can ask the user to set up a rule in Outlook to forward to the addresses.

Open outlook > Home > Rules > Select Manage Rules & Alerts
Select New Rule > Select Apply rule on message I receive located near the bottom of the list, then click Next.
Click Yes when asked This rule will be applied to every message you receive.
On the next list selected, the actions redirect them to people or public group and stop processing more rules
Click the underlined phrase people or public group in the bottom part of the window.
Type the email address to forward mail to in the To field, then click OK.
Select Finish
Or, in the admin center, create a distribution group, add the addresses to it, and then set up forwarding to point to the DL using the instructions in this article.

Don't delete the account of the user whose email you're forwarding or remove their license! If you do, email forwarding will stop.

you can also click the link here to learn to do that

You can convert it to shared mailbox even when syncing from AD, it will not revert back. Once the conversion is done, you can remove the license. Forwarding and other features will work fine, you will only need to assign a license to the shared mailbox if you use the Online archive, any hold functionality or the quota goes beyond 50GB.