[pve-devel] [PATCH proxmox 1/2] notify: smtp: forward original message instead nesting

[Wolfgang Bumiller]

On Wed, Dec 13, 2023 at 05:42:00PM +0100, Lukas Wagner wrote:
> For mails forwarded by `proxmox-mail-forward` to an SMTP target, the
> original message was nested as a 'message/rfc822' message part.
> Originally this approach was chosen to avoid having to rewrite
> message headers.
> Good email-clients, such as Thunderbird can display these inline.
> Other, more limited clients will show these messages as an attached
> .eml file, which is not really a good user experience.
> 
> This patch changes the approach for message forwarding to be more like
> forwarding mails in a mail client. We create a new message and
> add the original message body as a body. Additionally, we also copy
> over all message headers that are relevant to correctly display the
> original message body (e.g. Content-Type, Content-Transfer-Encoding)
> 
> Tested with a couple of different email messages (varying in
> structure, body parts, encoding, etc.) against the following SMTP
> relays:
>   - gmail
>   - outlook
>   - our own webmail service
> 
> Originally reported in our community forum:
> https://forum.proxmox.com/threads/proxmox-mail-forward-sends-mails-as-eml.137710/
> 
> Signed-off-by: Lukas Wagner <l.wagner at proxmox.com>
> ---
> 
> Notes:
>     proxmox-mail-forward needs a bump once this is applied.
> 
>  proxmox-notify/src/endpoints/smtp.rs | 99 ++++++++++++++++++++++++----
>  1 file changed, 88 insertions(+), 11 deletions(-)
> 
> diff --git a/proxmox-notify/src/endpoints/smtp.rs b/proxmox-notify/src/endpoints/smtp.rs
> index 064c9f9..4b8ff2d 100644
> --- a/proxmox-notify/src/endpoints/smtp.rs
> +++ b/proxmox-notify/src/endpoints/smtp.rs
> @@ -1,8 +1,9 @@
> +use std::time::Duration;
> +
>  use lettre::message::{Mailbox, MultiPart, SinglePart};
>  use lettre::transport::smtp::client::{Tls, TlsParameters};
>  use lettre::{message::header::ContentType, Message, SmtpTransport, Transport};
>  use serde::{Deserialize, Serialize};
> -use std::time::Duration;
>  
>  use proxmox_schema::api_types::COMMENT_SCHEMA;
>  use proxmox_schema::{api, Updater};
> @@ -231,17 +232,93 @@ impl Endpoint for SmtpEndpoint {
>              }
>              #[cfg(feature = "mail-forwarder")]
>              Content::ForwardedMail { ref raw, title, .. } => {
> -                email_builder = email_builder.subject(title);
> +                use lettre::message::header::{ContentTransferEncoding, HeaderName, HeaderValue};
> +                use lettre::message::Body;
>  
> -                // Forwarded messages are embedded inline as 'message/rfc822'
> -                // this let's us avoid rewriting any headers (e.g. From)
> -                email_builder
> -                    .singlepart(
> -                        SinglePart::builder()
> -                            .header(ContentType::parse("message/rfc822").unwrap())
> -                            .body(raw.to_owned()),
> -                    )
> -                    .map_err(|err| Error::NotifyFailed(self.name().into(), Box::new(err)))?
> +                let parsed_message = mail_parser::Message::parse(raw)
> +                    .ok_or_else(|| Error::Generic("could not parse forwarded email".to_string()))?;
> +
> +                let root_part = parsed_message
> +                    .part(0)
> +                    .ok_or_else(|| Error::Generic("root message part not present".to_string()))?;
> +
> +                let raw_body = parsed_message
> +                    .raw_message()
> +                    .get(root_part.offset_body..root_part.offset_end)
> +                    .ok_or_else(|| Error::Generic("could not get raw body content".to_string()))?;
> +
> +                // We assume that the original message content is already properly
> +                // encoded, thus we add the original message body 'Binary' encoding.
> +                // This prohibits lettre from trying to re-encode our raw body data.
> +                // lettre will automatically set the `Content-Transfer-Encoding: binary` header,
> +                // which we need to remove. The actual transfer encoding is later
> +                // copied from the original message headers.
> +                let body =
> +                    Body::new_with_encoding(raw_body.to_vec(), ContentTransferEncoding::Binary)
> +                        .map_err(|_| Error::Generic("could not create body".into()))?;
> +                let mut message = email_builder
> +                    .subject(title)
> +                    .body(body)
> +                    .map_err(|err| Error::NotifyFailed(self.name().into(), Box::new(err)))?;
> +                message
> +                    .headers_mut()
> +                    .remove_raw("Content-Transfer-Encoding");
> +
> +                // Copy over all headers that are relevant to display the original body correctly.
> +                // Unfortunately this is a bit cumbersome, as we use separate crates for mail parsing (mail-parser)
> +                // and creating/sending mails (lettre).
> +                // Note: Other MIME-Headers, such as Content-{ID,Description,Disposition} are only used
> +                // for body-parts in multipart messages, so we can ignore them for the messages headers.
> +                // Since we send the original raw body, the part-headers will be included any way.
> +                for header in parsed_message.headers() {
> +                    let header_name = header.name.as_str();
> +                    // Email headers are case-insensitive, so convert to lowercase...
> +                    let value = match header_name.to_lowercase().as_str() {
> +                        "content-type" => {
> +                            if let mail_parser::HeaderValue::ContentType(ct) = header.value() {
> +                                // mail_parser does not give us access to the full decoded and unfolded
> +                                // header value, so we unfortunately need to reassemble it ourselves.
> +                                // Meh.

urgh

> +                                let mut value = ct.ctype().to_string();
> +                                if let Some(subtype) = ct.subtype() {
> +                                    value.push('/');
> +                                    value.push_str(subtype);
> +                                }
> +                                if let Some(attributes) = ct.attributes() {
> +                                    for attribute in attributes {
> +                                        value.push_str(&format!(
> +                                            "; {}=\"{}\"",
> +                                            attribute.0, attribute.1
> +                                        ));

should be more efficient to use
let _ = write!(value, "...");
instead of `push_str()` on a temporarily allocated (format!()) string.

> +                                    }
> +                                }
> +                                Some(value)
> +                            } else {
> +                                None
> +                            }
> +                        }
> +                        "content-transfer-encoding" | "mime-version" => {
> +                            if let mail_parser::HeaderValue::Text(text) = header.value() {
> +                                Some(text.to_string())
> +                            } else {
> +                                None
> +                            }
> +                        }
> +                        _ => None,
> +                    };
> +
> +                    if let Some(value) = value {
> +                        match HeaderName::new_from_ascii(header_name.into()) {
> +                            Ok(name) => {
> +                                let header = HeaderValue::new(name, value);
> +                                message.headers_mut().insert_raw(header);
> +                            }
> +                            Err(e) => log::error!("could not set header: {e}"),
> +                        }
> +                    }
> +                }
> +
> +                message
>              }
>          };
>  
> -- 
> 2.39.2