Exchange size reference
Email signature size for a Microsoft 365 transport rule: every design, measured
When a Microsoft 365 transport rule adds a signature to outgoing mail, Exchange Online limits how long that signature can be. This page explains the limit and lists how long each of Bantusign's email signature designs is against it, so you can pick a design that fits before you build on it.
The limit, and what counts toward it
Three things to know before you read the numbers.
The limit
Exchange Online rejects a transport-rule disclaimer longer than 5,000 characters. The count is of the signature's HTML, not of the words a reader sees: every tag, every inline style, every link and every image address counts. That is why a signature that looks short can still be long, and why an email signature built from nested tables, a photo, social badges and a button runs longer than a plain text one.
Bantusign's budgets
- Design target4,000
- Warning above4,500
- Exchange refuses above5,000
Bantusign keeps its own budgets under the limit so there is room for a real footer, links and a campaign banner: a design target of 4,000 characters, and a warning above 4,500. The complete rule also has a size limit of its own, 8 KB, which is not measured on this page.
Which delivery methods it applies to
The transport-rule limit applies only to that delivery method. The Outlook add-in inserts the prepared signature itself, so no transport-rule or Gmail size limit applies to it, and its compatibility is checked separately. Gmail has a limit of its own, 10,000 characters on each person's prepared signature.
What the numbers on this page are
Each design is measured on a sample profile: the library's preview layout drawn with a sample logo, no company footer, no legal disclosure and no campaign banner. As drawn is that. Every block on is the same design with each optional block switched on (photo, social badges, booking button, tagline and the rest), so it is the longest that design gets without a footer, disclosure or banner. Your own signature will differ, because your people, links and footer are not the sample's. Bantusign's editor measures your configured signature for real before you publish.
The full library, measured
Every design, measured
99 designs, from 2,278 to 4,884 characters as drawn (median 3,189). As drawn, 98 fit with room to spare, 1 fit with little room and 0 are over the limit; 87 are within Bantusign's 4,000-character target. With every optional block on, 98 of 99 measured designs fit with room to spare, 1 fit with little room and 0 are over.
Fits: up to 4,500 characters. Fits, little room: above 4,500 and up to 5,000. Over the limit: above 5,000. Figures come from Bantusign's public design catalogue and are updated when a design changes.
Using it
Browse the email signature template library to preview any of these designs, or read how a signature reaches your people to see how the transport rule, the Outlook add-in and Gmail each deliver one.