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Limits & quotas

Every limit Mailway enforces, and what happens when you reach it. Tier features and add-ons are on Plans & add-ons, and prices will be published at launch; this page is the engineering view.

Plan quotas

Section titled “Plan quotas”
FreeProTeamBusinessEnterprise
Emails / month1,00010,00040,000100,000custom
Emails / day100no capno capno capno cap
Throughput / month250 MB2.5 GB10 GB25 GBcustom
Retention30 days90 days180 days1 yearcustom
Projects1515unlimitedunlimited
Sender domains (team)11075unlimitedunlimited
Sender domains per project12510custom
Providers per project135unlimitedunlimited
Team members131025custom

When you hit them:

  • Email volume — Free stops at the limit (402 monthly_quota_exceeded); every other plan keeps sending (soft overage) and the team email gets a notice at 80% and 100%. Soft overage is not billed during the private beta; how it is billed will be published with launch pricing.
  • Data volume (total message bytes) — the other plans get a grace margin past the limit, then sends hold (402 throughput_quota_exceeded) until the month rolls over, an add-on, or an upgrade; Free holds at the limit.
  • Free daily limit — additionally, Free sends at most 100 messages per UTC day (429 daily_send_cap_exceeded). It limits abuse, is never billed, and resets at 00:00 UTC.
  • One Free team per person — the Free allowances (monthly mails, the daily limit and the share link) belong to the oldest Free team a person created. A newer Free team can be set up and tested in the sandbox, but it can’t send real mail or create a share link until it is on a paid plan (402 free_team_limit; over SMTP, 550). Accounts whose addresses differ only by dots count as one person. Upgrading or deleting the older team moves the allowances to the next one.
  • Quotas reset with the calendar month (UTC). Sandbox sends count toward nothing.

Message limits

Section titled “Message limits”
Limit Value Over it
Recipients per message (to+cc+bcc) 50 422 — split the audience or use batch
Subject 998 characters 422 (RFC 5322 line limit)
html / text body 2 MB each 422
Attachments per message 20 422
Single attachment (decoded) 10 MB 422 attachments:too_large
All attachments combined (decoded) 25 MB 422 attachments:total_too_large
tags 10 × 64 chars of [A-Za-z0-9_-] 422
Distinct tags per project 1,000 for sends (reusing a tag always works; a rule’s tag action isn’t limited) 422 tags:limit_reached
metadata 10 keys · 40-char keys · 500-char values 422
Idempotency-Key 12–40 chars of [A-Za-z0-9_-] 422
Batch elements per request 100 422
scheduled_at horizon ≥ now · ≤ 72 h ahead 422 scheduled_at:too_soon / :too_late

Mail arriving over the SMTP relay isn’t shaped by the API validator — but provider recipient caps still apply, which is where the over-cap fan-out takes over.

Rate limits

Section titled “Rate limits”
Surface Limit Keyed by
Send endpoint, authenticated 600 requests / minute your API key
Send endpoint, unauthenticated 60 requests / minute client IP
Batch endpoint, authenticated 6 requests / minute your API key
Batch endpoint, unauthenticated 2 requests / minute client IP

Per-key (not per-IP) on purpose: tenants sharing an egress IP — NAT, serverless — never throttle each other. Throttled requests get a 429 with a Retry-After header; back off and retry, with an idempotency key so the retry can’t double-send. A batch request counts once against the batch bucket (6/min), and each accepted element counts against your quotas.

API stability

Section titled “API stability”

The surface is versioned in the path (/v1/) and changes are additive-only: new optional fields and new endpoints may appear; existing fields, semantics, and error codes don’t change meaning within v1. Branch on error.code, never on error.message.