Thunderbird changes the size of attachment

Question

I want to send an email with an attached pdf file using Thunderbird. The file is about 15~16MB, but Thunderbird shows it as having 21MB:

This prevents the mail to be sent:

Any ideas what can be happening and how to solve it?

Thanks for any hint!!

(I know about file links, I just cannot use them for this particular issue.)

Answer 1

Nothing wrong here. Attachments are encoded in Base64. With headers etc. this adds about 33-37% to the original file size.

When you create a message you see original sizes of attachments; but global size limit later applies to the whole message, encoded attachment(s) included.

This limit you see comes from your e-mail server. Compare How can I send a large attachment?

You can:

• Compress the file
• or/and split it into parts and send separately.
• Use account on a different server (with higher limit) and hope it won't hit a lower limit somewhere on its way to the recipient (it may pass through several mail transfer agents, then there's a final server). The best case is when you and the recipient use the same server and its limit is high enough for you.