Strange characters after sending Newsletter

Hi,

I get Strange characters after sending Newsletter from admin..

It looks as..

*Newsletter and Product Notifications Manager
Please wait .. sending emails ..

Please do not interrupt this process! "mÿÿ"Ù¶”¢Äôü<‡ÄäT;PŁ&ÿÿ"Ùä¢ôä¢ÄªJ3s‡Œüp…„X¥öÿÿ"=nr3ÍÌÌ,--!/˜’@;ÿÿ"ÙŽ¤¢R`ÊN-ʯ46Åf vy Mÿÿ"Ù¦¼ÌŒÌPë?/ i ª8Ðdÿÿ"=¦‹3LòK±y«4Ðÿÿ"=>ò³3s’ ¬[Ò1c‡ ]ÿÿ"Ù®âÄÒÔ¢¤lþÁšÿÿ"=ÌòRSŠÊS“ôR"Eh.ÿÿ"£<É-XHÅ>˜’@;*

Customers receive email without any error..
Any idea what or where is the problem?

When you reply - scroll up and read about posting - provide the additional information

Hi kobra,

Here info about server...

Zen Cart v1.3.9h
Server OS: Linux 2.6.32-5-amd64
PHP Version: 5.3.3-7+squeeze13 (Zend: 2.3.0) PHP Memory Limit: 128M
Database: MySQL 5.1.63-0+squeeze1
HTTP Server: Apache

Installed via ftp : v1.3.9h [2011-11-21 13:17:13] (Fresh Installation)
saw this probleem few weeks ago... nothing big changed on site...

Website use engl. language

Use FTP and sort files by changed date and look for files altered around the time you suspect that this showed up

Also, do you have any addons installed?
What are they?

tnx kobra... i will upgrade Zen Cart to newer version.. so maybe the problem will gone... need to do this anyway

tnx for your time