Understanding MAPI and Aliases

By default, grommunio such as Microsoft Exchange (on-premises and Exchange Online) does not allow users to send messages from any of their secondary (alias) email addresses. Instead, Exchange typically requires that email be sent from either the primary SMTP address of a mailbox or from another mailbox/address for which the user has explicit "Send As" or "Send on Behalf of" permissions. Here are the key reasons why "sending as an alias" can be problematic and how Microsoft has approached this limitation:

Behavior in grommunio (and Exchange)

  • Single Primary SMTP Address: In traditional Exchange environments (and by

default in Exchange Online), each user mailbox has one "primary" SMTP address. That address is the one used when you send an email.

  • Alias Addresses: Mailboxes can have multiple "proxy" or "alias" addresses,

but these aliases exist primarily for receiving email. By default, Exchange does not allow you to send out through an alias.

From Microsoft’s standpoint, alias addresses are designed to ensure that a mailbox can receive email sent to multiple addresses or domains. Security and administrative considerations also come into play: Exchange wants to be explicit about which identity a user is sending from. Also, there are many other technical reasons, one of them is that MRs (meeting requests) also have the attendees identifiable through SMTP addresses. Using different aliases with meeting requests would therefore be prone to errors in matching these objects beyond the now-point-in-time view (for example switching aliases at different times).

Alternative workarounds

If you are using an on-premises deployment of Exchange (2016, 2019, etc.), out-of-the-box "Send from Alias" is also not currently supported, just like with grommunio.

Possible workarounds include:

  • Shared/Resource Mailbox: Create a separate mailbox or shared mailbox that has the desired alias as its primary address, then grant "Send As" permissions to your user. You can then select that mailbox as the From address.

  • Third-Party Tools or Transport Rules: Some organizations use custom transport rules or third-party utilities that rewrite the From header to appear as an alias. This is more complex, can be tricky to maintain, and may introduce mail-flow side effects.

With grommunio, we traditionally use Postfix as our main MTA (which is built-in with every appliance) and the recommended MTA. It is possible to create transport rules on that basis, like you can see based on the following example with a fixed recipient.

Alias transport rule in Postfix

Important

Please note to validate your transport maps appropriately, as they directly manipulate your MTA configuration. Take care and validate both rewriting and non-rewriting mail routing to work as expected.

In /etc/postfix/main.cf you can enable header checks with:

header_checks = pcre:/etc/postfix/header_checks

and use the appropriate header_checks file with (example):

/^(From:.*myuser@mydomain\.at)(\r?\n(.*\r?\n)*?^To:.*special@destination\.com)/m
    REPLACE From: [email protected]

After that, do not forget to create the appropriate postfix map:

postmap /etc/postfix/header_checks

How it works: When Postfix processes the headers of an outgoing email, it checks for a From line matching myuser@mydomain.at. It then looks ahead in the next lines for a To line matching special@destination.com. If both conditions are met in that single message header, it rewrites the From header to alias@mydomain.at. If the message is going to any other recipient, no rewriting occurs.

Of course, this is just an example, there are many possibilities to extend this by for example replacing the sender or the recipient(s) in a more generalized way.

Summary

By default, grommunio acts the same way like Microsoft Exchange in this case. Main Reason for that is the MAPI core technology which is the foundation of both.

Historically, aliases were for receiving mail only, and Microsoft locked down sending behavior for clarity, security, and auditability. However, the newer Microsoft 365 Exchange Online feature "Send from Alias" can be enabled by an admin to address this limitation in the cloud. This is done by rewriting addresses in the mail transport queue in M366. For on-premises Exchange, administrators generally need to rely on workarounds (such as shared mailboxes or distribution groups with "Send As" privileges). With grommunio you can use our provided Postfix to define generalized alias transports or use the same methods as common with Microsoft Exchange (and many other solutions).