fix(routing): spurious warning when user DefaultMailbox is set to "Default"

[Copilot]

When a user's DefaultMailbox is configured as "Default" (the implicit fallback), the server logged a warning on every received message because it searched for "Default" in the explicitly configured mailboxes list — where it never appears, since it's auto-created at runtime.

Change

In Smtp4devServer.cs, skip the configured-mailbox lookup when userDefaultMailbox equals MailboxOptions.DEFAULTNAME:

// Before
if (!string.IsNullOrWhiteSpace(userDefaultMailbox))

// After
if (!string.IsNullOrWhiteSpace(userDefaultMailbox) &&
    !string.Equals(userDefaultMailbox, MailboxOptions.DEFAULTNAME, StringComparison.OrdinalIgnoreCase))

The warning Mailbox 'Default' was not found. Falling back to default mailbox is now only emitted when a user references a genuinely missing non-default mailbox name.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>User is set with Default as mailbox, logging [WRN] Mailbox 'Default' was not found. Falling back to default mailbox</issue_title>
<issue_description>No mailboxes created, only a User. Mail is received and relays fine, but each time this logs as Warning:

• [WRN] Mailbox 'Default' was not found. Falling back to default mailbox

In case it matters, "Deliver messages to sending user's default mailbox" is checked.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes User is set with Default as mailbox, logging [WRN] Mailbox 'Default' was not found. Falling back to default mailbox #2067

---

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[rnwood]

@copilot The build triggered by this PR has failed. Please use the Azure DevOps REST API to investigate and fix the issues.

🚨 IMPORTANT: Only proceed if this is the most recent commit on your branch. If a newer commit exists, ignore this notification.

Build Details:

• Build ID: 5878
• Build URL: https://dev.azure.com/rnwood/smtp4dev/_build/results?buildId=5878 (for reference)
• Commit: eb34989
• Build Status: FAILED

Instructions for @copilot:

1. First verify this is the latest commit - If eb34989 is not the most recent commit in this PR branch, ignore this notification

2. Use Azure DevOps REST API to get build details and failures:
   `�ash

   Get build overview

   curl -s "https://dev.azure.com/rnwood/smtp4dev/_apis/build/builds/-version=6.0" | jq '{status: .status, result: .result, sourceVersion: .sourceVersion}'

   Get failed tasks and their logs

   curl -s "https://dev.azure.com/rnwood/smtp4dev/_apis/build/builds/5878/timeline?api-version=6.0" | jq '.records[] | select(.result == "failed") | {name: .name, task: .task.name, errorCount: .errorCount, logId: .log.id}'

   Get specific error details from a failed task log (replace LOG_ID)

   curl -s "https://dev.azure.com/rnwood/smtp4dev/_apis/build/builds/5878/logs/LOG_ID" | grep -i "error|failed"
   `

3. Analyze the error messages from the API responses to identify compilation errors, test failures, or other issues

4. Make targeted fixes based on the specific error messages found

5. Build locally after making fixes to verify the solution

The Azure DevOps REST API provides comprehensive build failure information without requiring browser access.

[Copilot]

@copilot The build triggered by this PR has failed. Please use the Azure DevOps REST API to investigate and fix the issues.

**🚨 IMPORTANT: Only proceed if this is the most recent commit on you...

Investigated the build failure. The failing test is E2ETests_Pop3_RetrieveAnd_Delete.RetrieveThenDeleteMessage(pop3Mode: "ImplicitTls"), which fails with:

[FTL] SMTP server failed to start. TlsMode: "StartTls", Port: 25, AllowRemoteConnections: true, Exception: CryptographicException

This is a pre-existing CI infrastructure issue — the same failure occurs in other unrelated PRs (e.g., build 5877 for PR #1608). It's caused by a TLS certificate not being available in the Windows CI environment, and is not related to the change in commit eb34989.

[rnwood]

@copilot The build triggered by this PR has failed. Please use the Azure DevOps REST API to investigate and fix the issues.

🚨 IMPORTANT: Only proceed if this is the most recent commit on your branch. If a newer commit exists, ignore this notification.

Build Details:

• Build ID: 5889
• Build URL: https://dev.azure.com/rnwood/smtp4dev/_build/results?buildId=5889 (for reference)
• Commit: eb34989
• Build Status: FAILED

Instructions for @copilot:

1. First verify this is the latest commit - If eb34989 is not the most recent commit in this PR branch, ignore this notification

2. Use Azure DevOps REST API to get build details and failures:
   `�ash

   Get build overview

   curl -s "https://dev.azure.com/rnwood/smtp4dev/_apis/build/builds/-version=6.0" | jq '{status: .status, result: .result, sourceVersion: .sourceVersion}'

   Get failed tasks and their logs

   curl -s "https://dev.azure.com/rnwood/smtp4dev/_apis/build/builds/5889/timeline?api-version=6.0" | jq '.records[] | select(.result == "failed") | {name: .name, task: .task.name, errorCount: .errorCount, logId: .log.id}'

   Get specific error details from a failed task log (replace LOG_ID)

   curl -s "https://dev.azure.com/rnwood/smtp4dev/_apis/build/builds/5889/logs/LOG_ID" | grep -i "error|failed"
   `

3. Analyze the error messages from the API responses to identify compilation errors, test failures, or other issues

4. Make targeted fixes based on the specific error messages found

5. Build locally after making fixes to verify the solution

The Azure DevOps REST API provides comprehensive build failure information without requiring browser access.