Receiving email on alias but not able to send any emails - Outlook on the web?

I had that happen as well when I was trying to send an email from my mobile Outlook app. The emails would show up in my sent file as if they’d gone through; however, after I checked my emails on Outlook.com, I would return to my mobile app to discover that the emails had disappeared from my Sent file entirely and hadn’t gone through at all. It only happened when I tried to send from third party aliases in the mobile app and the Microsoft specialists thought it was directly related to the fact that Microsoft no longer supports third party aliases.

I have a similar issue, I believe it to be with the Outlook365 client.

I have email that is through gmail, that I use O365 as my client. My gmail is actually an email alias domain. (aliasdomain.com).

I have created the domain in my google suite and can receive email fine (in O365), but when I send email form O365, it defaults to the primary domain.

I have ticked the Exchange server as suggested.

I have created the Send From as suggested.

Here is my twist.

When I log into GMAIL Web interface, I can send from ******@aliasdomain.com, fine, no problem. So I dont believe it is on gmail side.

From O365, when I Send from ******@aliasdomain.com it goes out and shows up being from ******@primarydomain.com

I can confirm that this problem persists and it doesn't seem like there's any way to send emails from aliases anymore. On both Outlook for the web and Outlook for Mac (new), we can select an alias to send from, but the email is always sent from the primary address. This forum is full of auto-generated and AI-generated answers claiming to show how to do it, but you follow all the steps and it still doesn't work. My company is moving to using "shared mailboxes" (one per individual) as a way to do this, which is quite a workaround but could suffice. It's inexplicable and frustrating that Microsoft doesn't want to support email aliases anymore, it's been a widely used feature for more than 25 years and they won't even announce this policy change or give a reason.

I used to have Microsoft 365 Email Essentials through GoDaddy, which allowed me to set up aliases directly with GoDaddy. For some reason, these aliases worked seamlessly from the start, both in Outlook for Mac and Outlook for iPhone. They appeared in the dropdown list, and I could easily select them on my iPhone while on the go.

However, since GoDaddy restricts domains associated with their Microsoft 365 Email Essentials from being linked with Azure tenants, I switched to Microsoft 365 directly from Microsoft. While this switch allows me to utilize the full capabilities of my domain, I’ve noticed an issue: I could no longer select the aliases I'd added in the Microsoft 365 admin center. These aliases didn’t appear in the dropdown list in Outlook for Mac, Outlook for iPhone, or even Outlook on the web.

So, I began searching the forums.

1. I discovered that, by default, the option to send emails from aliases is turned off in Microsoft 365 from Microsoft. To enable this, I had to go to the Microsoft 365 Exchange Admin Center at https://admin.exchange.microsoft.com/#/settings, select Settings -> Mail flow, and manually turn on sending from aliases.
2. After that, I needed to adjust my settings in Outlook Web. I went to the View tab -> View Settings -> Mail -> Compose and Reply -> scroll down a lot -> find the section Addresses to Send from -> check all the necessary aliases in the list.
3. I also noticed that sending an email from another mailbox to any new alias helps Outlook recognize it, and it will start offering it as an option to select afterwards.

Thank you so much Alex, this was really helpful, just what I was looking for.