New Exchange Online Feature Supports Sending Email from Proxy Addresses

TonyRedmond 

I have tested this new functionality and it works when sending from a proxy address, but it seems like it doesn't work for a proxy address on a Shared Mailbox. 

I have a Shared Mailbox called SharedMailbox1, it has Primary SMTP address mailto:SharedMailbox1@domain1.com and proxy smtp address mailto:Sharedmailbox1@domain2.com. User1 has Delegate access and Send As permissions from SharedMailbox1.

User1 can Send As mailto:sharedmailbox1@domain1.com, but when trying to send email as mailto:sharedmailbox1@domain2.com I receive a message saying I don't have permissions.

Any idea or is this expected behavior? 

Thanks!  

The fact we're still waiting for this basic feature years later (added to the roadmap over 2 years ago) really isn't good enough.

I've just migrated a company onto our system, they don't yet have their laptops and are using office.com to get started and need to be able to send from their existing addresses which have been added as alias addresses.

That such basic yet critical functionality is missing really is just not on. Microsoft, please get on with it.

Roadmap Feature ID: 59437

Q&A: How to Send Email Using Proxy Addresses with Exchange Online

Exchange Online supports the ability to send email using any SMTP proxy address assigned to a mailbox. Following the announcement of the feature, users had many questions including what clients can be used. Here are some common questions and answers about the feature, including some PowerShell to report the set of proxy addresses assigned to user mailboxes.

https://office365itpros.com/2021/04/27/send-email-using-proxy-addresses-with-exchange-online/