Contains the parameter of EXPIRE capability.
This read-only property contains the parameter of EXPIRE capability reported by the server. The value of the property is valid only after CAPA command has been sent either automatically or from code (see AutoCAPA property description) and only if ExtEXPIRESupported property is true.
The value of -1 indicates that messages never expire.
For more information see RFC 2449, Section 6.7.