"in this case no need to implement whole TCP/IP stack as its already implemented on the PC, MCU can communicate its mail destination and mail contents to the PC via serial port"

Yes, that is certainly another (and simpler) way of going about it - assuming that a suitable PC is available nearby (in which case, why not do the whole thing on the PC?)

But Raj was actually asking about having the 8051 send the e-mail itself - he said nothing about PCs.
So; it's over to Raj - is this a viable alternative for you?