Developer traces two DKIM failures to SMTP relay rewriting email bodies and Date headers

A developer running several sites on one Ubuntu VPS found DKIM failing despite passing local key checks, because a shared SMTP relay altered messages after signing. The first problem was long 8bit lines in contact-form emails being re-wrapped, which changed the body hash; switching the body to quoted-printable encoding resolved it for most domains. A second failure persisted even with a matching body hash, and was traced to the relay replacing the signed Date header with its own formatted version. The author's tests showed that letting Postfix add the Date header avoided the mismatch, while manually set Date values failed. The post recommends comparing body hashes against receiver ARC records and diffing sent versus received headers to locate signature breakage.