Use the -Replace operator and zero-width assertion, i.e. 'lookahead' and 'lookbehind', in the RegEx.

sal ss select-string -ea 0
# all addresses by SessionId
ss (ss \@eurocollege.nl MailLog |
% {$_.line -replace '.+(?<=\()(\w+)(?=\)).+','$1.+RCPT TO'} |
sort -unique) MailLog |
% {$_.line -replace '.+(?<=RCPT TO(<.+>).*','$1'}

# showing SessionID:
$sessionIDs = ss \@eurocollege.nl MailLog |
% {$_.line -replace '.+(?<=\()(\w+)(?=\)).+','$1'} | sort -unique
$sessionIDs | % {
$_
ss "$_.+RCPT TO" MailLog |
% {$_.line -replace '.+(?<=RCPT TO(<.+>).*','$1'}
}

--
Kiron