Although settlement letters are inadmissible to prove liability or amount, they are admissible 'when the evidence is offered for another purpose.' FED. R. EVID. 408. In particular, such correspondence can be used to establish an independent violation unrelated to the underlying claim which was the subject of the correspondence. See Eisenberg v. University of N.M., 936 F.2d 1131, 1134 (10th Cir. 1991) (affidavit obtained in settlement negotiations admissible to impose Rule 11 liability); Urico v. Parnell Oil Co., 708 F.2d 852, 854-55 (1st Cir. 1983) (evidence of settlement negotiations admissible to show interference with efforts to mitigate damages); Resolution Trust Corp. v. Blasdell, 154 F.R.D. 675, 681 (D.Ariz. 1993) (finding admissible evidence of settlement negotiations used to prove retaliatory motive); see also 23 CHARLES ALAN WRIGHT & KENNETH W. GRAHAM, JR., FEDERAL PRACTICE AND PROCEDURE § 5314, at 282 (1980) ('Rule 408 is  inapplicable when the claim is based upon some wrong that was committed in the course of settlement discussions; e.g., libel, assault, breach of contract, unfair labor practice, and the like.').