emails received garbled after upgraded php to 8.1 and PHPMailer from 6.0.8 to 6.8

[sonhoangnz]

Hello,

We just updated our app, php from 7.2 to 8.1 and PHPMailer from 6.0.8 to 6.8
Emails were sent and received fine for most users. However, there is only one client email app Outlook - email content was garbled

I compare email headers before/after upgrade for the affected client

Old email header

...
x-mailer: PHPMailer 6.0.7 https://github.com/PHPMailer/PHPMailer
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0

This is a multi-part message in MIME format.
--b1_HRK3Q97IZYLLSohrkD2PhLxFzZwa4h8wKQD1cKrzc
Content-Type: text/plain; charset=3Dus-ascii

Body content ....
...

New email header (after the upgrades)

....
x-mailer: PHPMailer 6.8.0 https://github.com/PHPMailer/PHPMailer
Content-Type: text/plain; charset="Windows-1252"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0

o=95=E5=C5N=99=93=84=F06QLx=B0=B3=AE=05>=B6=B9=04=18=C9W=C8!=99=EF6=80=A8=
=9E=D7=A7=B5<=A9z=D7=B1=B7=FAej)=DC=85=AA=ECz=B1=AB=1C=8A$^=AA=E7=AC=B4=DA8=
...

It can be seen that the whole session below is missing from the new email header:

This is a multi-part message in MIME format.
--b1_HRK3Q97IZYLLSohrkD2PhLxFzZwa4h8wKQD1cKrzc
Content-Type: text/plain; charset=3Dus-ascii

Also the body is encoded with strange chars. Therefore, email body was garbled

Furthermore, I checked the email headers in Thunderbird before and after upgrade - the content was displayed correctly:

Before the upgrade

X-Mailer: PHPMailer 6.0.7 (https://github.com/PHPMailer/PHPMailer)
...  <- Some extra header here
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="b1_HWANsRj89baCQksHkr6ZVtYYfJ2179c7ShAX06vuaI"
Content-Transfer-Encoding: 8bit

This is a multi-part message in MIME format.
--b1_HWANsRj89baCQksHkr6ZVtYYfJ2179c7ShAX06vuaI
Content-Type: text/plain; charset=us-ascii

After the upgrade

X-Mailer: PHPMailer 6.8.0 (https://github.com/PHPMailer/PHPMailer)
...  <- Some extra header here
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="b1=_FVXduDlLl9VYPEQSxdKR4ZCdwOJvJYCe8Aw2SBM"
Content-Transfer-Encoding: 8bit
...
  <- A empty line here
--b1=_FVXduDlLl9VYPEQSxdKR4ZCdwOJvJYCe8Aw2SBM
Content-Type: text/plain; charset=us-ascii

I knew that the text This is a multi-part message in MIME format was removed from 6.8 as mentioned in #2938. However, I do not know why there is no boundary value for the outlook client. Could you please help

[Synchro]

Since you're only seeing this for one recipient, the thing to double-check is that the message they receive is the same as the one you sent. So send them a message and BCC it to yourself, then get an exact copy of what they received, and compare the two. They should be identical (apart from received headers). If there is a difference (like that missing section), you know it's happening somewhere in the mail chain between your script and their inbox.