Spam detection fails when subject contains emojis

[Jaggob]

FreeScout's spam detection based on subject line patterns (e.g., [SPAM]) fails to correctly identify and mark emails as spam when the subject line contains emoji characters.

Steps to Reproduce

1. Configure spam detection settings to mark emails with [SPAM] in subject as spam
2. Receive an email with subject: [SPAM] Regular text without emojis → Works correctly, marked as spam ✓
3. Receive an email with subject: [SPAM] 🎉 Text with emoji → Fails, not marked as spam ✗

might be related to UTF-8 encoding issues with multi-byte characters in the pattern matching logic?

Environment

• FreeScout Version: 1.8.195
• Spam-Filter Module: 1.0.29
• PHP Version: 8.3
• Server: Ubuntu 24.04
• Database: MySQL
• Web Server: Nginx

[freescout-help]

Can you share Subject: header of such email.

[freescout-help]

And are you using "Filter by Subject"?

[Jaggob]

Here's the Subject header from an email that failed to be detected as spam:

Subject: [SPAM] =?utf-8?q?=F0=9F=8E=81Dringend:?= Du verpasst riesige
	Belohnungen!

Yes, I'm using Filter by Subject, "[SPAM]" is the only phrase / line I have entered there..

Maybe it's not only the Emoji but (also) the multi-line Subject headers that causes trouble?

[freescout-help]

We could not reproduce the issue. Are you fetching via IMAP or POP3?

[Jaggob]

I'm fetching via IMAP.

The problem only seems to occur when the mail server adds [SPAM] to the subject line and the Subject header has mixed plain-text and encoded UTF-8 segments. ([SPAM] as plain text, followed by encoded UTF-8 with emoji (e.g., [SPAM] =?utf-8?q?...?=))

Tried to replicate by sending an E-Mail with [SPAM] + Emoji in Header and it was correctly detected as spam ([SPAM] is part of the encoded string itself (e.g., =?utf-8?Q?=5BSPAM=5D...?=)):

Subject: =?utf-8?Q?=5BSPAM=5D_=E2=8F=B0_Test_ob_der_Spam_Filter_funktionie?=
 =?utf-8?Q?rt?=

[freescout-help]

Also works fine.

Try to check the "Subject:" field using this method: https://github.com/freescout-help-desk/freescout/wiki/Fetching-Emails#checking-eml-file