The Problem
The default icon looks out of place especially on macOS Tahoe.
Proposed Solution
I created an icon that mostly matches the macOS Mail icon except with Mailspring colors. You can download this PNG and then generate the .icns file using this website or the commands below.
mkdir mailspring.iconset
sips -z 16 16 mailspring-1024-padded.png --out mailspring.iconset/icon_16x16.png
sips -z 32 32 mailspring-1024-padded.png --out mailspring.iconset/icon_16x16@2x.png
sips -z 32 32 mailspring-1024-padded.png --out mailspring.iconset/icon_32x32.png
sips -z 64 64 mailspring-1024-padded.png --out mailspring.iconset/icon_32x32@2x.png
sips -z 128 128 mailspring-1024-padded.png --out mailspring.iconset/icon_128x128.png
sips -z 256 256 mailspring-1024-padded.png --out mailspring.iconset/icon_128x128@2x.png
sips -z 256 256 mailspring-1024-padded.png --out mailspring.iconset/icon_256x256.png
sips -z 512 512 mailspring-1024-padded.png --out mailspring.iconset/icon_256x256@2x.png
sips -z 512 512 mailspring-1024-padded.png --out mailspring.iconset/icon_512x512.png
cp mailspring-1024-padded.png mailspring.iconset/icon_512x512@2x.png
iconutil -c icns mailspring.iconset
To apply it, open the Mailspring app info and drag the icns file into the icon spot.
Alternative Solutions
Update the packaged icon, but I don’t think that would fit for other platforms.
Context
macOS Tahoe introduces Apple’s new Liquid Glass style of icons, which makes old icons look out of place.