FreeTierMail

Send transactional email from .NET through several providers' free tiers as one sender. Each message goes to a provider with quota left, and a failure at one provider moves the message to the next without sending it twice.

Status: alpha. The API can change until 1.0.

Why

A small project that sends sign-in links, receipts and alerts can stay on free tiers for a long time, but each tier is small and resets on its own clock. Together the permanent free tiers below come to roughly 900 messages a day. FreeTierMail spends them in the right order, keeps a share for the mail that matters most, and routes around a provider that is down.

What it does

• Quota-aware routing. Daily and monthly quotas per provider, with their reset times, are counted and reserved before each send. By default the quota that resets soonest is spent first: a daily quota that resets tonight goes before a monthly one that must last three more weeks.
• Failover without double sends. On throttling, a used-up quota, an outage or a problem with the account (a revoked key, an unverified domain), the next provider is tried. When a request went out and no answer came back, it is not: the result is Unknown, unless you opt in.
• Critical mail first. EmailPriority.Critical messages try the providers you mark first, and may use a reserve (10 percent of each quota by default) that normal mail cannot touch.
• Idempotency. The same IdempotencyKey within 24 hours returns the first result without sending. Resend also gets the key in its own header.
• Honest results. Every send returns what happened at each provider it tried.
• Usage. GetUsageAsync() returns used, limit and reset time per quota, and the freetiermail.quota.remaining gauge exports the same.

What it is not for

Marketing campaigns, bulk mail, or getting around a provider's limits. Use one account per provider per sending domain. Opening several free accounts at one provider to multiply its quota usually breaks that provider's terms.

Providers

Three packages:

• FreeTierMail: the mailer, quota tracking, AddFreeTierMail() for dependency injection, and every HTTP provider below. The providers add no dependency, and Native AOT trimming drops the ones you do not register.
• FreeTierMail.Smtp: any SMTP relay, through MailKit, for a provider without an API or a paid fallback. Separate so only SMTP users download MailKit.
• FreeTierMail.Testing: a fake provider and the contract tests every provider passes.

Provider	Namespace	Free tier, checked 2026-09-26
Brevo	FreeTierMail.Brevo	300 a day, shared by marketing and transactional mail (from 2026 reviews; Brevo's page did not load for us)
Mailjet	FreeTierMail.Mailjet	6,000 a month, 200 a day
Resend	FreeTierMail.Resend	3,000 a month, 100 a day
Mailgun	FreeTierMail.Mailgun	100 a day, one custom domain
Elastic Email	FreeTierMail.ElasticEmail	3,000 a month, 100 a day
SMTP2GO	FreeTierMail.Smtp2Go	1,000 a month
MailerSend	FreeTierMail.MailerSend	500 a month, 100 API requests a day
Any SMTP relay	FreeTierMail.Smtp (package FreeTierMail.Smtp)	whatever your relay allows

Free tiers change without notice. The numbers are configuration, never constants: set Daily and Monthly to what your plan says. SendGrid has had no free plan since May 2025, so it is not listed.

Install

dotnet add package FreeTierMail --prerelease

Add FreeTierMail.Smtp only if you send through an SMTP relay.

Send a message

using FreeTierMail;
using FreeTierMail.Brevo;
using FreeTierMail.Resend;

using var http = new HttpClient { Timeout = TimeSpan.FromSeconds(30) };
var mailer = new FreeTierMailer(
[
    new BrevoProvider(http, new BrevoOptions { ApiKey = brevoKey, Daily = 300 }),
    new ResendProvider(http, new ResendOptions { ApiKey = resendKey, Daily = 100, Monthly = 3000, PreferForCritical = true }),
]);

var result = await mailer.SendAsync(new EmailMessage(
    from: new EmailAddress("links@example.org", "Example"),
    to: [new EmailAddress("rider@example.com")],
    subject: "Your sign-in link")
{
    TextBody = "Open this link to sign in: https://example.org/verify?token=...",
    Priority = EmailPriority.Critical,
    IdempotencyKey = "sign-in:4f1c",
});

switch (result.Status)
{
    case SendStatus.Sent: /* result.Provider accepted it */ break;
    case SendStatus.Unknown: /* a provider may have sent it; do not resend on your own */ break;
    case SendStatus.Failed: /* nothing went out; result.Attempts says why at each provider */ break;
}

Create one FreeTierMailer per set of accounts and keep it: it holds the quota counts. Give the HttpClient a timeout and no retry handler; the mailer decides retries.

With dependency injection

builder.Services.AddFreeTierMail(builder.Configuration.GetSection("FreeTierMail"))
    .AddBrevo()
    .AddResend()
    .AddMailjet();

Quotas, keys and the provider order go in configuration. The wiki covers the rest:

Page	Covers
Configuration	The configuration shape, keys, switching providers per environment, a local inbox, Gmail
Bounces and complaints	The suppression list and the webhook for each provider
Production	DNS (DKIM, DMARC, SPF), a shared quota store for several instances, tracing and metrics

Security

• API keys are never logged, traced, put in an exception or printed by ToString.
• EmailAddress accepts one bare address and refuses line breaks, so a header cannot be smuggled in through an address, name or subject.
• Webhooks are verified before their body is read, with constant-time comparison; a call that fails the check changes nothing. A suppression prints no address.
• Report a vulnerability as SECURITY.md describes, not in a public issue.

License

MIT.