Overview
Mailbox delegation in Exchange Online allows a user to send emails using another mailbox identity. This is commonly used for shared mailboxes, departmental mailboxes, or executive delegation. There are two main delegation types:
Send As
Recipient sees the email as coming directly from the mailbox.
Send on Behalf
Recipient sees:
User Name on behalf of Mailbox Name
This article also covers group-based delegation using mail-enabled security groups.
⭐ Use Cases
Use Cases for “Send As”
The identity of the individual sender is hidden; communication appears to come directly from the mailbox.
✔ Suitable for:
Support teams sending from support@company.com
Sales teams sending proposals from sales@company.com
Finance sending invoices from billing@company.com
HR sending from hr@company.com
✔ Why use it:
Maintains a single, professional team identity
Avoids exposing the personal identity of staff
Ideal for customer-facing shared mailboxes
? Example:
Rohit sends using support@contoso.com → Customer sees support@contoso.com only.
Use Cases for “Send on Behalf”
The receiver sees exactly who initiated the email.
✔ Suitable for:
Executive assistants replying on behalf of a director
Department heads sending communications for teams
When accountability and transparency are required
Temporary delegation scenarios
✔ Why use it:
Shows who actually performed the action
Good for EA → CEO workflows
Meets compliance requirements in some industries
? Example:
Rohit sends for ceo@contoso.com → Recipient sees:
Rohit Yadav on behalf of CEO Contoso
Use Cases for Group-Based Permissions
Using a mail-enabled security group (MESG) simplifies permission management for large or frequently changing teams.
✔ Suitable for:
IT helpdesk teams
Support centers (rotating shifts)
Sales teams with high staff turnover
Organizations with strict access control workflows
✔ Why use it:
Add/remove users from the group → permissions update automatically
No need to modify each shared mailbox manually
Easier, cleaner, and scalable administration
? Example:
Add Helpdesk-MESG to a mailbox’s Send As + Full Access.
→ All group members get permissions instantly.
Step 1 — Open the Exchange Admin Center
Sign in with admin credentials.
Navigate to Recipients → Mailboxes or Recipients → Shared depending on the mailbox type.
Step 2 — Configure “Send As” Permission
Assign to users:
Select the mailbox.
Open Mailbox Delegation.
Under Send As, click Add.
Select the user(s).
Save.
Assign to groups:
✔ Must be a mail-enabled security group
❌ Microsoft 365 Groups are not supported
❌ Normal security groups (non-mail-enabled) not supported
All members inherit the Send As right.
Step 3 — Configure “Send on Behalf”
Select the mailbox → Mailbox Delegation.
Under Send on Behalf, click Add.
Select users or a mail-enabled security group.
Save.
Step 4 — Expected Outlook Behavior
Send As
Appears as the mailbox itself
May take up to 2 hours for replication
Works in OWA, Outlook Desktop, and Mobile
Send on Behalf
Shows: User Name on behalf of Mailbox Name
Reflects immediately in most cases
Step 5 — How Users Send Mail
Outlook Desktop
New Email → Show From
Select mailbox
Send
Outlook Web
New message
Select ••• → Show From
Choose mailbox
Troubleshooting
Send As Not Working
Wait up to 2 hours
Re-add permission
Confirm group is mail-enabled
Send on Behalf Incorrect
Verify correct permission type
Restart Outlook if cached
Group Permissions Not Applying
Check group is mail-enabled security group
Check new members’ replication time (30–60 minutes)
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