Plugin Name

Please check whether or not you fill in the email address of General > Settings. I strongly recommend you put another email address to 'From' box of plugin settings — ideally an address on your own site's domain, since email sent with a 'From' address on a domain you don't control (e.g. a free gmail.com/icloud.com address on a server that isn't Google's/Apple's) is increasingly likely to be rejected by that provider's spam filters. Leaving the 'From' field blank so it matches your admin email's domain is usually the safest choice.

This almost always means the message left your server successfully but was rejected afterwards by the receiving mail provider. Check your host's mail delivery log (many hosts, including Sakura, provide one in the control panel) for a bounce message. A bounce mentioning something like "554 5.7.1 rejected due to local policy" means the receiving provider (e.g. Apple/iCloud) refused it — this is a deliverability/reputation issue between your server and that provider, not a bug in this plugin. Trying a different recipient address, or sending through an authenticated SMTP service (e.g. via a plugin like WP Mail SMTP), usually resolves it.

Both services shut down permanently (im.kayac.com in 2017, NMA in 2018) and their old API endpoints simply time out or 404. Rather than leave dead, silently-failing options in the settings screen, they were removed in version 0.60. Prowl, Pushover, and Email remain fully supported.