Send email from server without creating new Office 365 user account/mailbox

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We've just moved to Office 365 as it's cheaper than our previous email provider. Email addresses all work for our users, now we need more control.

We need to send automated emails from our backend. We don't want to pay for a new user seat for each email address we might need to send as. How can we configure the Exchange server or Office 365 to allow us to do so?

Best Answer

The "From" and "Reply to" addresses, when sent via SMTP, can be literally anything you want them to, as long as you use SMTP authentication from your backend via a single account. I don't believe that O365 will let you configure anonymous relay.

Of course, to send email, you don't need to send through your O365 instance at all. You could just, you know, send the mail. Your backend applications could send mail directly, or you configure an SMTP relay on your network for your backend to use, and then that relay would just send the mail to the intended recipients.

Or as EEAA suggested, you can use a hosted email relay service.

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Documented maximum number of users you can have connected to an Office 365 user mailbox

I received the following feedback from Microsoft with regards to throttling in place for Office 365:

The throttling limit for attribute “RCAMaxConcurrency”, which defines the number of concurrent connections allowed from Outlook, is set to 40 which translates to roughly to roughly 14 max conncections from Outlook 2007/2010/2013 clients to the mailbox.

However, Microsoft also recommended a workaround for this would be to use a Shared mailbox which would get around this problem as Shared mailboxes are not handled the same as User mailboxes on Office 365.

Hope this helps somebody else in the future!

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OK I'm not totally happy with this, but I got it working. I feel like there's a more straightforward way though, but I can't figure it out.

After further research, I found that virtual_alias_maps is run before anything in transport_maps, which is why I thought it was being ignored. So if you have addresses being rewritten then whatever the output from the virtual alias rewrites needs to match what you're going to feed into transport_maps. Then my transport_maps hash looks like this:

email@example.com        smtp:example-com.mail.protection.outlook.com
@example.com  :
*       :

This would cause email sent to email@example.com to be relayed to Office 365, but everything else gets delivered locally.

It works, but I still feel like there's a better way to handle this.

Best Answer

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