WP Debug errors

Hi, I would like to try this plugin out but am getting several errors on activation:

The plugin generated 6167 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

I have not poked around to see what those are, it’s been awhile since I’ve had to chase those down. More critically, after configuring my mailbox, I get a white screen of death with these errors when I click save on the options page:

Deprecated: whitelist_options is deprecated since version 5.5.0! Use apply_filters_deprecated instead. Please consider writing more inclusive code.

Deprecated: add_option_whitelist is deprecated since version 5.5.0! Use add_allowed_options() instead.

The debug stack for the first (I think) was triggered by the second, which in turn was triggered by line 255 in postie.php