Failure to file proof of service is a procedural irregularity, not a jurisdictional defect, and it may be cured by motion or sua sponte by the court. The court may grant this relief only upon such terms as may be just, and only where there is no prejudice to a substantial right of a party. The court may not make such relief retroactive, to the prejudice of the defendant, by putting the defendant in default as of a date prior to the order. A court may not give effect to a default judgment that, prior to the curing of the irregularity, was a nullity requiring vacatur.
Cherkassky v. Goykman, NY Slip Op 01904 (2d Dep't April 2, 2025)