feat(EmailWorkers): Implement email worker functionality

[nudded]

Closes #274

Looking for initial comments so I can wrap up this implementation. I've successfully validated reply and forward functionality.

example handler:

use uuid::Uuid;
use worker::*;

#[event(email)]
async fn main(message: EmailMessage, _env: Env, _ctx: Context) -> Result<()> {
    let message_id = message.headers().get("Message-ID")?.unwrap();

    let msg = format!(
        "From: Cloudflare bot <{}>
To: Toon <{}>
In-Reply-To: {}
Message-ID: <{}@redacted.com>
Subject: Email well received!

I've parsed the mail!

",
        message.to_email(),
        message.from_email(),
        message_id,
        Uuid::new_v4()
    );

    let new_message = EmailMessage::new(&message.to_email(), &message.from_email(), &msg)?;

    message.reply(new_message).await?;
    Ok(())
}

[DougAnderson444]

Does this part need to be a specific format? Or can it be any string?

let msg = format!(
        "From: Cloudflare bot <{}>
To: Toon <{}>
In-Reply-To: {}
Message-ID: <{}@redacted.com>
Subject: Email well received!

I've parsed the mail!

"

What I am wondering is if it needs a specific format, maybe it should use the type system to enforce it?

If it's just any old string, then disregard

[DougAnderson444]

What about an API like this:

use uuid::Uuid;
use worker::*;

#[event(email)]
async fn main(message: EmailMessage, _env: Env, _ctx: Context) -> Result<()> {

    let new_message = EmailMessage::try_from(
        RawEmailMessage::builder()
            .from_name("From Name")
            .from_email(&message.to_email())
            .to_name("To Name")
            .to_email(&message.from_email())
            .subject("Email well received!")
            .date("Sat, 15 Mar 2025 22:06:02 +0000")
            .message_id(format!("{}@redacted.com", Uuid::new_v4()))
            .in_reply_to(message.headers().get("Message-ID")?.unwrap())
            .message("I've parsed the mail!")
            .build(),
    );

    message.reply(new_message).await?;

    Ok(())
}

With the type system enforcing mandatory fields at compile time and removing any risk of user formatting errors?

[DougAnderson444]

Actually, on second thought, maybe msg should just be left a a string and if the user wants to use a build helper, it can exist in userland as a separate crate.

[nudded]

Actually, on second thought, maybe msg should just be left a a string and if the user wants to use a build helper, it can exist in userland as a separate crate.

yeah, In my testing not a lot of the existing crates seem to work well, so it might be useful to create a small crate that fills this gap (but should imho not be part of this crate)

[DougAnderson444]

This looked promising: https://docs.rs/mail-builder/latest/mail_builder/index.html

[nudded]

This looked promising: https://docs.rs/mail-builder/latest/mail_builder/index.html

Tried this, got a CPU limit exceeded error on Cloudflare. (I think it's related to it adding the Date header, but did not want to spend much more time debugging)

[nudded]

@zebp If you have some time for a review :)

[jeholliday]

I tried out these changes, and I was able to receive and reply to emails just fine. I also used the mail_parser and mail_builder crates, and I found that they worked pretty well in combination with workers. The one thing I noticed is that you have to explicitly set a date and message id when building a message because otherwise the defaults will panic when it tries to get the date or generate a random id.

It would be nice if functionality for the SendEmail binding could also be added. I tried adding it myself I was also able to send emails that way. I just reused the binding for an EmailMessage and it worked to call send().