According to Discord, Go was (or at least it used to) cause huge latency spikes because of it's GC - they've rewritten it in Rust and now it's A LOT faster, without any latency spikes.
Which is a factor that really does not matter for a test server
300ms latency spikes on a particularly pathological production workload are irrelevant for the stated use case of a test email server.
When does one decide whether an environment is a test? I would like to test SMTP notifications of a staging server at work, approximately 15 million emails to be sent. This is NOT production, but those 300 ms spikes would be unacceptable. That Rust rewrite seems to be a godsend.
The GC iterations they were talking about were during incredibly high throughout. Most users would not notice this. It's not some general indictment of Go.
The go GC has kept evolving since.
A tiny cross-platform binary, a tiny docker image image? That’s every bullet point but the written in rust one!