Email signature generator › Signature as attachment

Why your signature turns into an attachment

A paperclip on every message you send, containing a file called image001.png that you never attached. It is your signature logo, and what is happening is a deliberate behaviour rather than a bug.

What is actually happening

There are two ways to put an image in an email. It can be referenced by URL, so the recipient's client downloads it when the message is opened, or it can be carried inside the message itself as an embedded part.

An embedded image is technically an attachment with a flag saying "display this inline, do not list it as a file". Whether that flag is respected is up to the receiving client. Many honour it and show only the inline image. Others, including several webmail interfaces and most mobile clients, show the paperclip as well.

So the message is not broken, and nothing was added by mistake. The image is travelling with the message and one client is being literal about what that means.

Why your client embedded it

Usually because the signature was pasted from a source where the image was a local file or a data URI. Outlook is the most eager: pasting a signature that references a file on disk causes Outlook to embed a copy in every message, permanently, and rename it image001.png.

Apple Mail and Outlook for Mac do the same when a signature is copied from a page that inlined its images. Thunderbird does it deliberately when the signature file uses a relative path, which is a feature there rather than an accident.

The fix, if you want the paperclip gone

Host the logo at a public https:// address and reference it by URL in the signature. Nothing then travels with the message, no paperclip appears anywhere, and the message gets lighter — which matters on long threads.

The cost is that the logo is subject to remote image blocking, so a share of your recipients will see the alt text instead. That is the whole trade: embedded always shows and always attaches; hosted never attaches and sometimes shows.

Doing it properly in Outlook

Editing the signature in place is not always enough, because Outlook keeps the embedded copy in its signature files. Delete the signature entirely, create a new one, and paste a version whose image tag points at the hosted URL. Then send yourself a test — if the paperclip is still there, the old signature is still being applied to that account, which happens when there are several accounts and only one was updated.

When to just leave it

If your recipients are mostly on desktop clients that honour the inline flag, the paperclip is invisible to them and only you are bothered by it. If your signature has to be readable by people who block images — auditors, government addresses, correspondents on privacy-focused providers — embedded is the more reliable choice and the paperclip is the price. Decide by who reads your mail, not by what looks tidy in your sent folder.

What it costs in weight

An embedded image is encoded into the message, and the encoding adds roughly a third to its size. A 30 KB logo becomes about 40 KB of message. On a single message that is invisible. On a thread of eight replies it is 320 KB, and Gmail clips messages at around 102 KB — so a long conversation with an embedded logo gets truncated for the recipient, hiding the older part of the thread behind a link.

This is the strongest argument for hosting rather than embedding, and it has nothing to do with the paperclip. If your signature carries an image and you have long threads with external correspondents, host it.

Telling which one you have, in ten seconds

Send yourself a message and open the raw source. An embedded image shows a message part with Content-Disposition: inline and a Content-ID, and the signature references it as src="cid:something". A hosted image shows no extra message part and the signature carries a plain src="https://...".

Anything starting with cid: is travelling with your mail. That is the whole diagnosis.

The case for keeping it embedded

There is one situation where the paperclip is the right price: correspondence that has to be readable years later, from an archive, without depending on a web server. A hosted logo breaks the day the URL stops resolving — a domain change, a reorganised site, a deleted bucket — and every message you ever sent retroactively loses its logo. An embedded image is part of the record and cannot rot.

For most people that does not matter. For a law firm or anyone whose sent mail is evidence, it does.

Frequently asked questions