This example should work. Copy and paste the part you need (remove mail or add mail).
$string = "Carl Marx (carl.marx@contoso.com); William Shakespeare (william.shakespeare@contoso.com); Paul Newman (paul.newman@contoso.com); Bruce Willis (bruce.willis@contoso.com)"
# Remove mail for each
$removemail = $string -split ';' -replace "\(.*\)",";"
$removemail.Trim()
# Result: Carl Marx ;
# Add mail for each
$addmail = $string -split ';' -replace "@",".mail@"
# Result: Carl Marx (carl.marx.mail@contoso.com)