Can a Message have multiple senders?

Question

When I want to check who send a specific email to me with JavaMail.

I can use

Message.getFrom()

which returns an

Message[]

In what circumstances can a revived Message have multiple Identities it came from?

Regarding the API this makes more sense for outgoing Emails.

So can I really on, that:

Address from = message.getFrom()[0];

Always gives me back exactly 1 Address which the Message was send from?

I have now Implemented something like:

Address[] fromAddress = message.getFrom();

if (fromAddress == null || fromAddress.length > 1) {
    // Don't Process the Email and Notify me
}

Answer 1

Yes, it seems that Message.getFrom() can really return multiple addresses.

I had a look at the sources for Apache Geronimo's implementation of the JavaMail API, and it will return multiple addresses if there are multiple From: headers, or multiple addresses inside one header.

As to whether this could happen:

As explained by Alex K.'s answer, the standard allows multiple "From" addresses if there is a single "Sender". I don't know whether anyone really sends mails with multiple "From" addresses, but it is standards-compliant.

Another situation which actually happens in practice:

Some spammers apparently send mail with multiple From: headers. This is not standards-compliant, but apparently some mail servers still accept the mail. This is apparently done to get past mail filters that filter by the From: address. The spammers include multiple addresses in the hope that a simple-minded filter will let the message pass if it finds one header with a "From"-address it likes.

So in summary: Yes, you should expect Message.getFrom() to return multiple addresses (or none at all). However, you probably don't need to expend a lot of energy for handling that case. Maybe you can even get away with just logging it as an error - that depends on your application.