Dark mode inversion
How mail apps recolour emails, including signatures, when dark mode is on.
In dark mode, some email clients leave messages alone, some darken only light backgrounds and some fully invert colours. The same signature can look different in each.
Dark logos on transparent backgrounds can disappear against a dark background. A thin light outline or a logo version on a solid white tile survives better.
Preview your signature in dark mode before rolling it out; Sendoff’s editor has a dark preview toggle.