Welcome to the forums,
There's a attribute on your account, that it seems we don't expose it in end-user preferences anymore, set to 'internet' which needs to get it set to type 'outlook' (ironic).
About the fastest way to correct this (short of having you do a SOAP ModifyPrefsRequest) may be to have an admin run this on your account:
zmprov ma
user@domain.com zimbraPrefMailSignatureStyle outlook
Or a an admin/domain admin can use the admin console to un-check the box in COS/Account > Preferences tab> "Use standard Internet signature style"
So we know what version you're running you can type this in the main search bar:
$set:get version
http://www.zimbra.com/forums/announc...html#post62754