.NET and xUnit

Setup

dotnet add package Mailcatchr.Sdk

Read the key and mailbox id from the environment or user secrets, never from source.

A shared fixture

public sealed class MailFixture : IDisposable
{
    public MailcatchrClient Client { get; } = new(new MailcatchrClientOptions
    {
        ApiKey = Environment.GetEnvironmentVariable("MAILCATCHR_API_KEY")
            ?? throw new InvalidOperationException("MAILCATCHR_API_KEY is not set"),
    });

    public string MailboxId { get; } = Environment.GetEnvironmentVariable("MAILCATCHR_MAILBOX_ID")!;

    public static string UniqueAddress(string label) =>
        $"{label}-{DateTimeOffset.UtcNow:yyyyMMddHHmmssfff}@acme-ci.mailcatchr.com";

    public void Dispose() => Client.Dispose();
}

The test

public sealed class PasswordResetTests(MailFixture mail) : IClassFixture<MailFixture>
{
    [Fact]
    public async Task Reset_email_contains_a_working_link()
    {
        var address = MailFixture.UniqueAddress("reset");
        await App.RequestPasswordResetAsync(address);

        var summary = await mail.Client.AwaitMessageAsync(mail.MailboxId,
            to: address,
            subject: "Reset your password",
            timeout: TimeSpan.FromSeconds(30));
        summary.Should().NotBeNull("the reset email should arrive within 30 s");

        var message = await mail.Client.GetMessageAsync(mail.MailboxId, summary!.Id);
        var link = Regex.Match(message.TextBody!, @"https://\S+/reset\?token=\S+").Value;
        link.Should().NotBeEmpty();

        var response = await App.HttpClient.GetAsync(link);
        response.StatusCode.Should().Be(HttpStatusCode.OK);
    }
}

AwaitMessageAsync returns null on timeout rather than throwing, so the assertion message can say what was expected.

Reading over IMAP instead

When the code under test is itself a mail client, read the mailbox with MailKit and per-mailbox credentials. See Mail clients.