Email Signature Deliverability

Deliverability is decided mainly by the sender's authentication and reputation: SPF, DKIM, and DMARC records on the domain, the sending infrastructure's history, and how recipients engage with the mail. A signature is a small part of the content filters see, but it is on every message, so a bad pattern repeats thousands of times. The signature elements most associated with filtering problems are images that make up most of the visible content, especially an image-only signature with no live text; very large or numerous images; links through public URL shorteners, which spammers abuse; link text that shows one domain while the link points to another; and tracking redirects on domains with a poor reputation. Authentication can break too. DKIM signs the message body, so a tool that appends a signature after the message has been signed invalidates the signature check unless it re-signs the mail. Practical guidance is simple: keep the signature mostly text, host a few small images, link to your own domain, avoid shorteners, and send test messages through a spam checker after any template change.