估计新邮件的大小

Question

We have a postfix server configured with message_size_limit=10240000 (the default) and I would now like to prevent the creation of any mail that is bigger than that. Because I get otherwise a "connection broken" error, which is difficult to present to the user in a meaningful error message.

The definition of message_size_limit is:

The maximal size in bytes of a message, including envelope information.

My question is: How can I calculate / predict / estimate the size in bytes of a created mail with lots of different attachments? javax.mail.Message.size() is only available when receiving a mail.

Answer 1

Take your draft Message object and use Message.writeTo to write it to a stream that does nothing with the data but just counts the number of bytes written:

public long getMessageSize(Message message) {
    CountingOutputStream counter = new CountingOutputStream(new NullOutputStream());
    message.saveChanges();
    message.writeTo(counter);
    return counter.getByteCount();
}

The CountingOutputStream is from commons-io , but the one from Guava or a slower ByteArrayOutputStream will do as well.

That will give you an estimate on the overall size of the message.

Doing this is going to be expensive so you can't (for example) do it for every character typed into the message by the user. You need to do it only on certain events (eg, adding an attachment) or after (eg) 5 minutes of typing data into a message. You need a strategy to reduce the cost of the operation while providing timely feedback to the user.

If you do this, be sure to call Message.saveChanges before each call to Message.writeTo , and before really sending the message.

Answer 2

One thing which some don't realize is that binary attachments are always base64 encoded, and occasionally, even ASCII attachments. Any attachment of K bytes will ultimately be 33% larger due to the encoding process. So you need to know if the folks that configure the server take that into consideration.

Otherwise, what appears to be an email less than the advertised maximum, can be substantially more.