Unexpected Output Error

I installed this plugin last in a series of plugins I usually install on my WP sites. The plugins are in order of installation:

1) Really Simple Captcha
2) Contact Form 7
3) All in One WP Security and Firewall

I get the following error:

The plugin generated 144 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.

https://wordpress.org/plugins/wp-mailto-links/