fix(smtp): preserve <br> line breaks in HTML emails

[uzkikh]

Context

HTML-only emails (e.g. from Synology DSM 7.3 Task Scheduler) use <br> tags for line breaks. The existing implementation passed the raw HTML body to bluemonday with AddSpaceWhenStrippingTag(true), which replaces every tag including <br> with a space. This caused all notification content to appear on a single unreadable line.

The ntfy notification before the fix

Task Scheduler has completed a scheduled task. Task: daily-backup Start time: Mon, 01 Jan 2026 02:00:00 +0000 Current status: 0 (Normal) From MyNAS

The ntfy notification after the fix

Task Scheduler has completed a scheduled task.

Task: daily-backup
Start time: Mon, 01 Jan 2026 02:00:00 +0000
Current status: 0 (Normal)

From MyNAS

Steps to reproduce

# Start ntfy with SMTP enabled
ntfy serve --smtp-server-listen :25 --smtp-server-domain example.com

# Send an HTML-only email with <br> line breaks
swaks \
  --server 127.0.0.1 --port 25 \
  --from sender@example.com \
  --to ntfy-mytopic@example.com \
  --header "Content-Type: text/html; charset=utf-8" \
  --body "Line one.<BR>Line two.<BR><BR>Line three."

# Check the notification — all text appears on one line:
#    "Line one. Line two.  Line three."

Root cause

The readHTMLMailBody() function used bluemonday.StrictPolicy().AddSpaceWhenStrippingTag(true) which replaces all tags including <br> with spaces, discarding line break semantics.

Fix

Convert <br> tags to newlines before passing to bluemonday. The regex covers all variants: <br>, <BR>, <br/>, <BR />.

Tests

• Updated TestSmtpBackend_HTMLOnly_FromDiskStation and TestSmtpBackend_HTMLEmail to reflect correct output
• Added TestSmtpBackend_HTMLEmail_BrTagsPreserved covering the Synology Task Scheduler email format

Related to #690

[binwiederhier]

This is a reasonable fix. Thank you @uzkikh and Cursor/Claude :-)