What is the significance of the exclamation mark (!) on ACARS page of a Boeing 737?
Okay guys, the verdict is in.

It is a symbol to indicate the crew alerting chimes are inhibited.
From OUT (off block) until OFF+15 and estimated ON-10 until IN (on block) the chimes to alert for new data and messages are suppressed to avoid nuisance warnings during critical flight phases.

Happy landings!