POP3: Request server capabilities
To get an idea what protocol features and extensions are supported by the POP3 server, it's needed to request its capabilities list. You can perform this request manually (set AutoCAPA property to capaNone and call GetCapabilities() method) or to let the class to do it automatically. In the latter case, using AutoCAPA property the class can be set to perform the request either after the connection is established, or after successful login to the server, or both. Please note that the allowed capabilities can differ for an anonymous user (before authentication) and for a known user (after successful authentication).
After the capabilities list is received, the known features and extensions are parsed automatically and the results are available using ExtXXXSupported properties. For example, to get an idea if the server supports the TOP command (this command allows to request message headers only, without a need to download messages completely), you need to check ExtTOPSupported property.
If you need to check an unsupported capability, you should create a handler for OnCapability event. TElPOP3Client fires this event for each received capability.