Print specs feel like a hazing ritual the first time you meet them. DPI, bleed, trim, safe zones, keep-outs. I have watched smart marketers get files bounced three times over details a prepress tech considers obvious. So here is the plain-English version of the three specs that cause virtually every rejection.
300 DPI: the sharpness floor
DPI is dots per inch, the print resolution. Screens get away with far less because they glow; paper does not. At 300 DPI, a 6 by 9 inch postcard needs artwork around 1875 by 2775 pixels once bleed is included. Anything much lower prints soft, and text is where softness shows first.
The classic mistake is stretching a small web graphic to postcard size. The pixel count does not increase when you scale up; you just print the blur larger. Start from the print dimensions and design at size.
Bleed: why your file is bigger than your card
Printers print on oversized sheets and trim down to final size, and industrial cutters have a tolerance of a millimeter or so. Bleed is the extra eighth of an inch of artwork extending past the trim line on every side, so that a slightly off cut still shows ink instead of a white sliver. For a 6 by 9 card, that means a 6.25 by 9.25 inch file.
If your background is a solid color or photo, extend it into the bleed. If your art arrives trim-sized, a good workflow can mirror the edges outward automatically, which is exactly what Relay's artwork upload does rather than bouncing your file.
Safe zones and the address panel
The same cutting tolerance that demands bleed also means nothing important should sit within about an eighth of an inch of the trim edge. That inner boundary is the safe zone: keep headlines, logos, phone numbers, and QR codes inside it.
Postcards add one more rectangle to respect: the carrier's address panel and postage area on the back. Artwork or QR codes placed there can be overprinted by the address block or barcode. This zone is not negotiable, which is why our editor renders it on the canvas and refuses QR placements that collide with it.
Three specs, one sentence each: design at 300 DPI at final size, extend backgrounds an extra eighth inch on all sides, and keep everything that matters away from the edges and the address panel. Get those right and your file sails through any printer's preflight, including ours.