To add a holiday banner to every employee's signature in Microsoft 365, create a campaign in ATOM Signatures, upload a banner 600 px wide, keep the audience on all signatures, set a start date and an end date, and publish. From the start date, the banner shows in every employee's signature; after the end date, it is gone, and nobody has edited a signature by hand.
What size and format should a holiday banner be?
Aim for 600 px wide. That is the figure ATOM recommends, and it matches the 600 × 200 px that Exclaimer recommends in its guide to seasonal email signatures, which says that size fits the Outlook reading pane width without scaling.
| Item | Recommendation | Source |
|---|---|---|
| Dimensions | 600 × 200 px (3:1 ratio) | Exclaimer |
| Width in ATOM | 600 px recommended, or 1200 px for a double-resolution (@2x) file; a warning appears above 700 px | ATOM console |
| Phones | Outlook for iOS and Android scale the banner to the device width: keep it readable down to 320 px wide | Exclaimer |
| Outlook for Windows | Can clip images wider than 624 px in the reading pane | Exclaimer |
| File weight | Exclaimer advises under 100 kB; ATOM accepts files up to 3 MB. Those are two different limits: the first is a recommendation, the second a ceiling | Exclaimer, ATOM console |
| Format | PNG, falling back to JPG (Exclaimer); ATOM accepts PNG, JPEG, GIF and WebP; SVG is not supported in mail signatures | Exclaimer, Microsoft Learn |
| Text contrast | At least 4.5:1 against its background | W3C, WCAG 1.4.3 |
| Alternative text | One descriptive sentence that names the offer, so the message survives when images are blocked | W3C, WCAG 1.1.1, Exclaimer |
| Link | Make the whole banner clickable, not only a button inside it | Exclaimer |
Double the numbers for a double-resolution file: 1200 × 400 px is the 600 × 200 px banner at twice the density. Do not rely on the picture alone for anything essential: when images are blocked, only the alternative text remains. For the other image problems, see Outlook signature image not showing.
How do I schedule the banner in ATOM Signatures?
A banner is a campaign. The wizard has four steps (Visual, Audience, Schedule, Review) and ends with Publish:
- In the ATOM console, open Campaigns and select New campaign.
- In the Visual step, upload the banner image (600 px wide, or 1200 px for a double-resolution file) or build it with the designer, then set the destination link.
- In the Audience step, keep All signatures to reach every employee, or open Refine (optional) and use Limit to these groups or people to target one team.
- In the Schedule step, set the Start and the End (optional), and turn on Also show on replies if you want the banner on replies.
- In the Review step, check the period and the audience, then try the real preview for an employee.
- Select Publish: the campaign stays scheduled until its start date, then the banner shows in the signature and disappears after the end date.
Visual. The banner can be an uploaded image or built in the designer (the Design tab). One to three variants (A/B/C) are possible; for a single holiday message, one is enough. The destination link is where a click on the banner leads.
Schedule. The shortcuts Right now, In 1 week and In 1 month fill the start date. The end date is optional and can be removed again with Remove end date. Exclaimer suggests refreshing seasonal banners every four to six weeks, which is a useful yardstick for how long a holiday campaign should run.
Review. Enter an employee's address in the real preview and select Test to see the signature that person would get. Then send yourself a real email before the start date: Exclaimer makes the same recommendation, because rendering differs between clients.
How do I show the holiday banner to only one team?
In the Audience step, the default is All signatures: every signature in the organization can show the banner. To narrow it:
- Choose signatures. Pick Chosen signatures and tick the ones that should carry the banner. Only people who have one of them see it.
- Refine by group or person. Open Refine (optional) and use Limit to these groups or people to keep, for example, the sales team.
- Exclude. Add targets under Exclude: they never see the banner, even when every signature is targeted.
This is the same targeting used for signatures themselves; the guide signatures by Microsoft 365 group explains how groups are used.
Only one banner is shown per email. If another campaign overlaps the same period and part of the same audience, the one with the higher priority is shown instead. The Review step warns you with "This campaign might be shadowed", and you can drag campaigns in the list to change their priority.
Does the banner show on replies and on phones?
By default a banner only appears on a new message. Turn on Also show on replies in the Schedule step if you want it there as well.
On phones, ATOM works in Outlook for iOS and Android with Microsoft 365 accounts (version 4.2352.0 or later); see Outlook signature on Mac and mobile. Exclaimer notes that those apps scale the banner to the device width, so test the image at 320 px wide before you publish.
What happens when the holidays are over?
With an end date, the banner stops showing on that date and nothing needs to be edited in January. Without one, the campaign stays active until you pause or archive it from the campaign menu. The click rate over the last 14 days appears on each campaign card, so you can see how the banner did.
If you also need a closure notice with dates and a contact, the post Holiday office closure email signature: templates has three ready-to-copy versions. For banners beyond the holidays, read the email signature as a marketing channel. To see how ATOM compares with another tool on campaigns, there is the ATOM vs Exclaimer comparison.
Related reading: the free banner size tool.
Frequently asked questions
What size should a holiday banner be in an email signature?
Exclaimer recommends 600 × 200 px, the width of the Outlook reading pane, and a file under 100 kB. ATOM recommends a width of 600 px (1200 px for a double-resolution file), shows a warning above 700 px and accepts a file up to 3 MB.
Can the holiday banner be shown to one team only?
Yes. In the Audience step, open Refine (optional) and use Limit to these groups or people. You can also exclude targets: an exclusion always wins, even when all signatures are targeted.
Does the banner disappear on its own after the holidays?
Yes, if you set an end date: the banner stops showing on that date. The end date is optional; without one, the campaign stays active until you pause or archive it.
Does the banner also show on replies?
Not by default: a banner only appears on a new message. Turn on Also show on replies in the Schedule step to have it on replies too.
Which image format should I use for the banner?
ATOM accepts PNG, JPEG, GIF and WebP images. Exclaimer advises PNG, falling back to JPG. Microsoft states that SVG files are not supported in mail signatures: use a PNG or a JPG.
Schedule your holiday banner in a few minutes. Banner campaigns are included with ATOM Signatures at no extra cost: CA$1 per user per month, plus taxes, with a 14-day free trial.
Sources
- Exclaimer — Seasonal email signatures and marketing campaigns (banner size, weight, format, alternative text, 320 px, 624 px, test send, refresh cadence): exclaimer.com
- Microsoft Learn — Office.Body, SVG files in mail signatures: learn.microsoft.com
- W3C — Understanding Success Criterion 1.4.3, Contrast (Minimum): w3.org
- W3C — Understanding Success Criterion 1.1.1, Non-text Content: w3.org
Sources consulted on November 2, 2026.