Build signup/login OTP in Axum on Tokio: OsRng, HMAC in DashMap/Redis/SQL, Tower rate limits, reqwest POST /emails with Idempotency-Key—contrast Actix extractors vs services; prefer HTTP over lettre; Rovela delivers only.
Build signup/login OTP in Actix Web: OsRng, HMAC-hashed codes in DashMap/Redis/SQL, rate limits, reqwest to POST /emails, and Idempotency-Key—prefer HTTP over lettre SMTP; Rovela delivers only.
Send transactional email from Rust with reqwest and the Rovela HTTP API, run concurrent sends behind a semaphore, retry with idempotency keys, attach files, relay over SMTP with lettre, and verify webhook signatures with HMAC.