Difference Between Uppercase SMTP and Lowercase smtp Proxy Addresses
Question details
The user wants to understand the distinction between uppercase SMTP and lowercase smtp entries in the Active Directory proxyAddresses attribute, and how to properly configure primary and alias email addresses.
- Product
- Microsoft Exchange / Active Directory
- Device & OS
- not provided
- Scenario
- Configuring email addresses and aliases using the proxyAddresses attribute in Active Directory for Microsoft 365 synchronization.
- Observed behavior
- Uppercase SMTP defines the primary email address, whereas lowercase smtp defines secondary email aliases. Emails sent to the aliases are routed to the primary mailbox.
Ensure you have Active Directory Users and Computers (ADUC) open and possess the appropriate domain admin or account operator privileges before modifying proxy addresses.
How to Configure Primary and Alias Email Addresses in ADUC
Correctly format the proxyAddresses attribute in Active Directory to define the primary reply-to address and any necessary secondary aliases.
The proxyAddresses attribute in Active Directory determines the routing of emails. When syncing with Microsoft 365, the uppercase 'SMTP:' prefix designates the primary email address used as the reply-to address. The lowercase 'smtp:' prefix is used for alternate email aliases. Any emails sent to the lowercase aliases will automatically route to the primary mailbox.
Open Active Directory Users and Computers (ADUC) on your domain controller. Ensure the 'Advanced Features' option is enabled under the View menu.
Navigate to the Organizational Unit (OU) containing the target user account, right-click the user, and select 'Properties'.
Go to the 'Attribute Editor' tab and scroll down to locate the 'proxyAddresses' attribute. Click 'Edit'.
Type 'SMTP:primaryemail@yourdomain.com' (ensuring SMTP is fully capitalized) and click 'Add' to set the primary address.
To add secondary aliases, type 'smtp:alias@yourdomain.com' (ensuring smtp is entirely lowercase) and click 'Add'. Click 'OK' to save your changes and wait for directory synchronization to complete.
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Frequently Asked Questions
Can I have more than one uppercase SMTP address in Active Directory?
No, a user or contact object should only have one uppercase 'SMTP:' entry in the proxyAddresses attribute. This determines the primary reply-to address. Having multiple uppercase SMTP addresses can cause directory synchronization errors or unpredictable email routing behaviors in Microsoft 365.
What happens if all proxy addresses use lowercase smtp?
If no uppercase 'SMTP:' address is defined, Microsoft 365 and Exchange may automatically promote one of the lowercase 'smtp:' addresses to the primary address, or fall back to the user's User Principal Name (UPN) or default routing address (e.g., alias@domain.onmicrosoft.com).
Will emails sent to a lowercase smtp alias show the alias as the recipient?
While the sender addresses the email to the lowercase smtp alias, once the email reaches the mailbox, standard replies sent by the user will always come from the primary uppercase SMTP address, unless specific 'Send As' permissions or alias-sending features are explicitly enabled in Microsoft 365.
How long does it take for proxyAddress changes to reflect in Microsoft 365?
Changes made in on-premises Active Directory will reflect in Microsoft 365 after the next Azure AD Connect synchronization cycle completes, which typically occurs every 30 minutes by default.
