Problem compiling libESMTP

["Chausson, Cedric (FR - Neuilly)" <cchausson deloitte fr>]

Hello all,

I got libESMTP to solve my balsa mail sending problem. Downloaded it did
./configure --enable-require-all-recipients, and it worked ok. Then I did
make, it started compiling but ended with an error message : 

In file included from client-crammd5.c:32:
hmacmd5.h:31:25: openssl/md5.h: No such file or directory
make[2]: *** [client-crammd5.lo] Error 1
make[2]: Leaving directory
`/home/Cedric/Installation/libesmtp-0.8.3.orig/crammd5'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory
`/home/Cedric/Installation/libesmtp-0.8.3.orig'
make: *** [all-redirect] Error 2

Obviously I'm missing a file. Question : how to get it ? I know I have
openssl installed on my machine but maybe I need a certain version ?
Anyother ways to solve the problem ?

Thanks in advance.

Cédric CHAUSSON