Popful Mail at Hardcore Gaming 101 - A great article about Popful Mail at classicgaming.com.
Popful Mail at The PC Engine Software Bible - A little review of the PC Engine version, with a few screenshots.
Popful Mail Lair - A little site mainly about the Japanese versions of the game, and some PC-88 screenshots.
Shard's Domain - Another little site that looks dead. All sections don't work, but it has some art and screenshots.
The Falcom Heaven - This site's in a language my browser doesn't show correctly. It's got nice pics of the different version covers, though.
Falcom's Wallpapers - Falcom's website has a couple of Popful Mail wallpapers. The site's all in Japanese, though, so I don't know if they have other material...
See my general links page for more video gaming links.