Outlook constraints

Outlook sets the rules for what a signature can contain. These are not Sigil’s limits, and no signature product can work around them. Designing within them from the start is easier than discovering them at publish time.

30,000 characters

A rendered signature must be no more than 30,000 characters of HTML.

The portal blocks a publish that would exceed it, which means you find out at your desk rather than through a report of broken signatures.

The check has limits worth knowing. It renders the template with the sample person, and it measures the template alone. A campaign banner and a compliance footer are added when a signature is sent, and so are real directory values, which can be longer than the sample ones. Nothing re-measures at that point. A template that passes with only a few hundred characters to spare can still reach Outlook over the limit, so keep a margin.

Attached images do not count toward this, because the limit applies to the markup rather than to the attachments. What does count is the markup itself, and table-heavy layouts add up faster than they look.

If you are close to the limit, the usual savings are collapsing unnecessary nested tables, removing repeated inline styles that could sit on a parent element, and cutting whitespace. A signature produced by exporting from a rich-text editor is often several times larger than it needs to be, because such editors tend to wrap each run of text in its own table.

No SVG

Outlook does not render SVG. A signature containing one shows nothing where the image should be. The image library refuses an SVG upload for that reason, with “SVG is not supported in mail signatures; use PNG or JPG”, so a vector logo needs exporting as PNG first.

Use PNG for logos and line art, JPG for photographs. Export at roughly twice the display size for sharpness on high-density screens, then set an explicit pixel width on the <img> so it displays at the intended size.

Images are attached, not hosted

Images are referenced as cid:name.png and attached inline by the add-in.

A hosted image, pointing at an external URL, is suppressed by Outlook’s default image blocking. Recipients see a broken image placeholder unless they explicitly choose to download pictures, which most people never do.

See images.

Layout with tables

Outlook on Windows renders HTML with Word’s rendering engine. Modern CSS layout does not survive it.

Use nested tables with explicit widths. Put styles inline on elements rather than in a <style> block. Size type, spacing and images in pixels rather than relative units. The designer compiles to this shape, which is one of the reasons to use it.

Rounded corners are square

Word’s rendering engine ignores border-radius. A rounded button renders as a rectangle in classic Outlook, and a circular profile photo renders as a square.

Both are still worth using if the majority of your recipients are elsewhere, since a square version of either is a reasonable fallback rather than a broken one. The designer says so at the point you choose it, so nobody discovers it from a recipient.

Mobile visibility on replies

On Outlook mobile, the signature on a reply is not visible until the compose window is expanded to full screen. The signature is applied; it is just not shown in the collapsed view.

This is documented Outlook behaviour and is regularly reported as a fault.

Other client behaviours worth knowing

A new message with no other edits will not autosave a draft, even though the signature was set. Outlook decides that an unmodified message is not worth saving.

OnNewMessageCompose does not fire for a message started from the iOS Share sheet, so a message begun that way gets no automatic signature.

Only one event-based add-in runs at a time. If your organisation has several deployed, they run sequentially.

Dark mode

Every client built on Outlook on the web can recolour the text and background colours a signature sets, and never touches its images. The compose editor recolours all of them. The reading pane has at times been gentler and changed only colours that fail a contrast check. See dark mode. Light text over a dark picture therefore disappears, and a faint grey disclaimer becomes hard to read.

Nothing in the signature can prevent it, so the fix is a colour choice or a change of layout. See dark mode.

Testing your design

Preview in the portal shows a template with sample data, or as a real mailbox in your directory, which is enough to check that conditional sections collapse correctly.

To see how a client actually renders it, send a test email and open it in the clients your organisation uses. Outlook on Windows is the one worth checking most carefully, since its rendering engine is the most restrictive.