an.alogo.us

Want monospaced fonts in Mailplane on your Mac? Try using this as a custom stylesheet, selectable via the Preferences:

.qIKyDc,
.mMl8gd,
.rSfjbb,
.iE5Yyc,
div.msg div.mb,
div.ArwC7c,
div.ckChnd textarea,
textarea.tb,
td.ct {
    font-family: Monaco, monospace !important;
    font-size: 12px !important;
}

Gathered from various net sources (such as this), and from poking around in existing stylesheets (such as this one). Easily customized.

UPDATE 3/8/2009: This appears to have been broken by a recent Gmail update; will have to track down the new correct CSS selectors…