I think this is the case with all email programs: encoding overhead adds 33% to the message size. If you're reaching the maximum message size allowed by your mail service, or wish to utilize resources more efficiently, e.g. sending a large file at different times, it's better to upload the file to a file hosting site and send the download link by email.

The extension of yEnc to email would decrease the overhead to 1-2%.

While it is possible to send attachments using 8-bit characters (by using the yEnc add-on), it is not recommended, because not all servers support this. As you never know which route a sent message takes from the sender to the recipient, you also cannot guarantee that the message doesn't pass such a mail server on the way, so the attachment might turn up unreadable at the other end.

This topic is very interesting because recently I did a test with Microsoft Outlook 2010 and in the case of this program, problem does not exist. While Outlook uses rounds to display the size of the attachment, the entire email is not incremented. I do not know what the situation is for other programs - I am switching between Thunderbird and Outlook 2010.

So, either I do a bad test, or so it was designed. :)

Try this test: send yourself a message, with attachments, from Outlook and TB, and receive the message with TB. Compare the size of the two messages by displaying the Size column. Any difference? I would be surprised if there were.

I did a test and two different sizes came out. Test procedure:

1. Sending an e-mail with attachment from Outlook to the account it's attached to, and a copy to the second account in TB. The attachment is 2.34 MB. Outlook reports 2 MB and TB is 3.3 MB.
2. Sending an e-mail with attachment from TB to the account it's attached to, and a copy to the second account in Outlook. Same attachment as above. TB reports 4,2 MB, a Outlook 3 MB.

The most interesting thing is that the difference is basically the same in both cases but to the disadvantage of TB.

re :Sending an e-mail with attachment from Outlook...The attachment is 2.34 MB. Outlook reports 2 MB.

Are you saying that Outlook is saying the email plus attachment is smaller than the attachment itself?

It seems to me that Outlook uses rounding to the whole (2,34 MB after rounding it will give us 2 MB) and hence a "weird" difference. At least i think so.

Looks to me like Outlook is displaying the size of the decoded attachment (the space it would occupy on your hard disk) and Thunderbird displays the size of the encode attachment (how much space it actually uses in your mail folder). There's nothing wrong with that, so it doesn't need to be corrected, but you might file a bug to request for an enhancement to also see the decoded attachment size.