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 stay under 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.

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.

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. Use pixel values rather than relative units. The designer compiles to exactly this shape, which is one of the reasons to use it.

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.

Testing your design

Preview in the portal shows the shape of a template with sample data, 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.