Fastmail vs Klaviyo

Fastmail (Open-source software developed at Fastmail.) and Klaviyo both appear on the Smoower Developer Ecosystem Index.

Klaviyo (rank #237) holds a narrow lead over Fastmail (rank #430) on the overall Smoower ecosystem score (48 vs 40). The gap of 8 points reflects composite signals across code, docs, community and reach.

On code quality (the state of repositories, tests, releases and polish), Klaviyo is ahead of Fastmail. On education (docs, guides and learning material for developers), Fastmail is ahead of Klaviyo. On community (issue response, PR reviews and discussion health), Klaviyo is ahead of Fastmail. On reach (how visible the ecosystem is beyond its own repos), Fastmail is clearly ahead of Klaviyo. On momentum (release cadence and how fast the ecosystem moves), Fastmail is ahead of Klaviyo.

Fastmail carries 6,316 GitHub stars across 38 public repos, with 12 repositories active in the last 90 days and 2 external contributors on record. Klaviyo shows 573 stars across 117 public repos, 23 active in the last 90 days and 17 external contributors. The star gap on its own does not decide the comparison, but Fastmail's footprint is roughly 11.0x larger, which usually shows up in downstream signals like inbound issues and third party integrations.

Fastmail is the stronger read for anyone weighting reach. Klaviyo looks better where code quality is the deciding factor. The table below breaks the scores down pillar by pillar; the linked profiles cover the underlying repos, docs and community signals in full.