How to cost effectively use different email addresses to send out via SMTP for individual machine status reports on Office 365 o365

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At many client locations, we have shadowprotect and other apps that want to send status reports / mail from a client machine. We've set up in our pop / smtp mail server on a shared hosting web server that each client sends from using SMTP.  That way we can see what client it comes in from.  And there's no extra cost for all these extra sending addresses (incoming mail like bounces and spam redirect to our general business address so these addresses get no mail).  

I don't want to have a common mailbox across all locations / clients. If a machine gets compromised and that email and password gets out, we have to touch each client machine to change the password.

Moving to office 365, there;s a cost for each mailbox / user that you want to send out FROM (yes, you can have loads of email addresses / aliases at no extra cost for incoming mail.  But if you want to send out with different credentials like and, that will take 2 different office 365 licenses, right? I was thinking that would be $4 / client / month... but now seeing kiosk offerings for $2 / month.  That kiosk plan lets you send out via smtp?

Is that the cheapest way to have different machines sending out under different email addresses / passwords? Or some other alternative you'd suggest?

Another way I was thinking is to keep 1 of our other domains we have on the shared hosting email server for the ability to create free pop / smtp accounts.

So rather than, we would set up (we bought the same domain for our business with .com and .net TLDs).  The .net website redirects to the 'real' .com website.  And we aren't using any .net email addresses).  But keeping that .net domain on the shared email server will save us that $2 / client / month?
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Distinguished Expert 2018
You can use any of the SMTP relay methods supported by O365 that dont require authentication:

It's not true that you need separate mailboxes for each address, while Exchange/Outlook by default will allow you to only send as the Primary SMTP address, there are workarounds for that. You can also use the "direct send" method by granting Send As permissions on different object (shared mailbox for example).

You can also use a third-party service such as SendGrid or MailChip or whatever, even some free mail appliance. Dont forget to adjust your SPF records in such scenario.
Thanks!!  So you are saying with 1 account / license on office 365, (lets use as the example)  you can send from a LOB or device and not have the same password on all the devices?

a few things:  updating the spf record... that requires the sender IP to be static, right? At least some of these locations have dynamic IP addresses.

looking at that link to the document, there's the line:

Username/email address and password    Enter the sign in credentials of the hosted mailbox being used

I'm trying to avoid that, right? It wants the email address / password?

option 2 - some locations have port 25 blocked.

option 3 - more complicated than it needs to be / not sure if I could pull off all the requirements.

Sendgrid / mailchimp? - that's for newsletters - sending from 1 address to many?  I am looking for sending from several (many) different email addresses to 1 receiving address.
Most Valuable Expert 2015
Distinguished Expert 2018

If you use any of the SMTP methods that dont require authentication, you dont care about passwords. You can use dynamic IP for the SPF or add an IP range instead of single IPs. Option 1 is not what you are looking for.

You can use SendGrid/MailChimp however you like, as long as you dont break their rules. Even for 1:1 situations.

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