Hello Ganesh,
you want negative lookahead with alternation, like this:
.+@(?!clientabc\.com$|xyzclient\.com$).+
Please note, though, that the above regex does not provide any POSITIVE guarantee that the input is a valid SMTP address as specified by RFC2821.
It just provides a negative guarantee that the input is NOT an SMTP address from domains clientabc.com or xyzclient.com.
Best regards,
Lambert