SMTP relay

[IanaMac]

Hi all we are a large software development house with hundreds of Virtaul environments we recently migrated our exchange environments from internal to 365.

we use an internal Linux SMTP server running Postfix to control the mail send from these environments. 

We have been testing this pointing the postfix to 

smtp.office365.com on Port 587, what we have discovered is that unless the send address need to have mailbox (can be shared) and the sending server address needs send as permission to that mailbox. so 

sending a test mail from server 1 via SMTP server called email address removed for privacy reasons

email address removed for privacy reasons  needs a mailbox with email address removed for privacy reasons having send as permissions

this seems a bit crazy as we could end up with hundreds of mail boxes

Can anyone help with better options

[#2]

I'm not sure i'm understanding your question very well, But,
The only way to send an email using an email address like (email address removed for privacy reasons) is to have at least shared mailbox for this address,
You can use mail alias, but this is mostly used for receiving not sending.

[#3]

Hi Ahmed
Thanks for the response, if I have a SQL server on our internal network and the mail sending address is email address removed for privacy reasons do i need a shared mail for that send address if i am sending via a mail relay server?
Can you advise what is the preferred method bearing in mind we have 100s of servers that send mails for Development and QA departments?
Many thanks and appreciate your time

[#4]

What you describe is a requirement for SMTP Auth as the method for sending email. You might want to try one of the other 2 methods for relaying with Exchange Online: https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/how-to-set-up-a-multifunction-de...