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Why Microsoft 365 Exports SMTP Aliases in Lowercase

Maira Mehtab Sep 30, 2026 869 views

Question details

The user wants to know why primary email addresses are labeled with uppercase SMTP while aliases use lowercase smtp in Microsoft 365, and whether aliases can be converted to uppercase.

Why Are SMTP Aliases Exported in Lowercase in Microsoft 365?

Product: Microsoft 365
Device & OS: not provided
Scenario: Managing Microsoft 365 email accounts and exporting SMTP address lists for users or mailboxes.
Observed behavior: Primary email addresses appear as uppercase 'SMTP', while alias addresses are automatically converted to and exported as lowercase 'smtp'.

Before you start

Before managing your email addresses, ensure you have administrative access to the Microsoft 365 admin center or Exchange admin center, as standard users cannot modify primary SMTP assignments.

Solution 1Recommended

Understand the Difference Between SMTP and smtp

Learn the system design logic behind uppercase and lowercase SMTP designations in Microsoft 365.

In Microsoft 365 and Exchange Server, the capitalization of 'SMTP' serves as a critical system identifier rather than a simple text formatting choice. Uppercase 'SMTP' strictly identifies the primary routing email address for a mailbox.

Conversely, lowercase 'smtp' designates a secondary address, also known as an alias. Because a mailbox can only have one primary address, you cannot convert all aliases to uppercase. Attempting to force an alias to uppercase without changing the primary address will cause the system to default it back to lowercase.

Verify primary address status

Look for the address marked with uppercase 'SMTP' in your exported list or Exchange admin center. This is your active primary address that dictates the sender identity.

Understand the Difference Between SMTP and smtp

Solution 2

Change the Primary Email Address

If you need a specific alias to have the uppercase SMTP designation, you must set it as the new primary address.

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Frequently Asked Questions

Why did my new alias change to lowercase smtp after I added it?

During the creation process, the interface might temporarily display 'SMTP' for a new alias, which can be confusing. However, once saved, the system automatically converts it to lowercase 'smtp' because the uppercase designation is strictly reserved for the primary address.

Can I make multiple email addresses uppercase SMTP?

No, Microsoft 365 only allows one primary email address per mailbox. Therefore, only one address can hold the uppercase 'SMTP' designation at any given time to ensure proper mail routing.

Will the lowercase smtp designation affect email delivery?

No, the lowercase 'smtp' designation is strictly for administrative identification within the system. Emails sent to your lowercase aliases will be delivered to your mailbox just as reliably as those sent to your primary address.

How can I view all my aliases and their SMTP status?

You can view the status of all email addresses by logging into the Microsoft 365 admin center, selecting Active Users, clicking on a specific user, and looking under the 'Aliases' or 'Manage email types' section.

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