[Zope] cannot change property of type lines permanently

Hi, I'm trying this: <dtml-call "calendar.mailaddresses.remove(REQUEST.mailaddress)"> where 'calendar' is a folder, 'mailaddresses' is a property of it (type is lines) and 'mailaddress' is a variable the DTML method received via an HTML form. After the request, the removed line entry seems to be gone. It's not shown in the ZMI or in my HTML site which shows me the mailaddresses. But if I restart Zope the removed line is there again. Also if I copy the folder, then the copy shows me the removed entry in its properties. Any suggestions? Thank you, Marcus