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How to set an HTML signature in Proton Mail

Proton Mail is one of the few consumer clients with an honest HTML source view for signatures. It is also built around blocking remote content, which changes how you should handle the logo.

Where it is

1. Settings → All settings → Identity and addresses.
2. Next to the address, Edit.
3. Switch the signature on. The editor has a toolbar and, behind the </> button, a source view where you can paste real HTML.

The source view is the part that matters. It means you can paste a table with inline styles and have it stored as written, rather than as whatever a rich-text editor decided your paste meant.

The image question is different here

Proton blocks remote content by default for the people reading your mail, and its own users are the most likely to have left that setting alone. A logo at an https:// address will therefore be blocked more often for a Proton audience than for a Gmail one — Gmail proxies and caches images and shows them by default.

Proton offers an alternative: you can embed the image so it travels with the message. In the signature editor, inserting an image through the toolbar's image button and choosing to embed it stores the image as part of the signature rather than as a link. The message gets heavier, and the image shows without the recipient allowing anything.

For a small logo this is the right choice on Proton specifically. On a client whose audience mostly allows remote images, hosting is lighter.

Alt text stops being cosmetic

When images are blocked as a default rather than an exception, the alt text is not a fallback — it is what most recipients read. alt="Acme" on the logo is the difference between a blank rectangle and your company name. Never leave it empty on a signature logo, and never write alt="logo", which tells the reader nothing.

Addresses and aliases

Each address, including aliases on a custom domain, has its own signature. Proton also appends its own promotional line on free accounts; that is separate from your signature and switched off on paid plans.

What a Proton audience changes about the design

Most signature advice assumes the recipient's client loads remote images without asking, because Gmail does. If a meaningful share of the people you write to are on Proton, Tutanota or a desktop client with images off, you are designing for a different default and the whole hierarchy of the signature changes.

The practical consequence is that no information may live only inside an image. A phone number rendered as part of a logo graphic, a job title baked into a banner, a social account represented only by an icon — all of that disappears for a reader with images blocked, and they will not think to turn images on for a signature. Text carries the content; the image is decoration on top.

Size, when the image travels with the message

Embedding solves the blocking problem and creates a weight problem. An embedded image is encoded into the message body, and the encoding adds roughly a third to the file size. A 40 KB logo becomes about 54 KB of message, on every message, in both your sent folder and the recipient's mailbox.

Keep an embedded logo under 20 KB. That is achievable for almost any wordmark: export at exactly twice the display size, use PNG-8 with a limited palette for flat colour or a well-compressed JPEG for anything photographic, and strip the metadata. If the logo will not go under 20 KB without looking bad, it is probably too detailed to be legible at signature size anyway.

Custom domains and multiple identities

Proton accounts on a custom domain can carry several addresses, each with its own display name and its own signature. The signature follows the address you send from, which is chosen in the compose window. If you send from the wrong identity the signature changes with it — a useful cue that you picked the wrong one, if you notice it.

Frequently asked questions