Why email signatures behave differently in shared mailboxes

When working with shared mailboxes in Outlook, two different access modes behave differently — and neither supports both the correct From address and email signature at the same time.

This is a limitation in how Microsoft handles shared mailboxes versus full accounts.
The behavior you’re seeing comes from two different Outlook modes.

Here’s what’s going on — and what you can do.

Shared with me

• Outlook treats the shared mailbox as an extension of the primary user.

Result:

• ✅ Signature add-ins work (they run in the context of the primary user)
• ❌ The From address defaults to the primary account unless the user changes it manually

Converted to full account

• Outlook treats the mailbox as a separate account.

Result:

• ✅ The From address is correct by default
• ❌ Signature add-ins do not run unless someone logs into that mailbox directly

In practice, Outlook forces a trade-off:

• Correct From address
• or automatic signature injection

Not both.

Practical workarounds

Workaround 1: Shared with me + Send As

Best overall compromise

• Keep the mailbox as Shared with me
• Grant Send As permissions (not “Send on behalf”)
• Train users to:
  • Always expand the From field
  • Select the shared mailbox once (Outlook often remembers this per conversation)

Pros

• Signatures work
• No extra licenses required
• Low technical overhead

Cons

• Relies on user behavior
• One missed click can result in the wrong sender address

Workaround 2: Full account with background login

Only suitable for small teams

• Convert the shared mailbox to a full account
• Assign a license
• Log into the mailbox directly in Outlook at least once

In some cases, this activates add-ins for that mailbox.

Reality check

• Behavior is inconsistent
• Can break without warning
• Not officially supported by Microsoft

Workaround 3: Hybrid approach using add-in (primary mailbox) + server-side (shared mailboxes)

Most technically reliable option

• Personal mailboxes use add-in signatures
• Do not use the Outlook add-in for shared mailboxes
• Shared mailboxes use a simple, static server-side signature

Pros

• Correct From address
• No client dependency
• Works consistently for shared mailboxes

Cons

• No client-side preview
• Limited personalization

Bottom line

There is no setup that delivers all of the following at the same time:

• Correct From address
• Automatic client-side signatures
• No user interaction
• No additional license

Recommended approach:

Stay with Shared with me and Send As, and clearly document the From-field behavior for users.