Beginning with New Defenders #137, the series changed the name of the letters column from Defenders Dialogue (as it appeared through #136) to Air Mail.
Incorporating Angel into the new Air Mail heading jazzed up the letters page and drew attention to the high-flying hero, who played more or less a supporting role within the series.
This design also harked back to early issues of the X-Men (#4-13, #15-22), which pictured Angel flying alongside the nameplate on the cover.