SC
EGT OVERLIMIT - there is no other "more severe ECAM" warning. This message requires investigation by the enigneer to establish time and temp over AMM limit, which appears to have happened in your case. As for the 423 degC, on our V2500-A5 a/c the start limit is 635 degC. Suspect you may be using brand B engine on your a/c?
Some engine FADEC and a/c logic has an "impending" exceedance limit. So the red-box appears on the flight deck screen, but the actual temp is not exceeded, the red box should clear when the EGT winds down, i.e. the box does not remain as a hard indication. Hope this helps.
Brgd's
N1 Vibes