Even though I routinely use OpenSSL as the SSL provider for Exim (which supports either OpenSSL or GnuTLS, compile-time decision), for *testing* I routinely use GnuTLS's gnutls-cli(1). It's way better than s_client(1) or socat(1) for development testing in protocol work, simply because the --starttls mode doesn't presume to handle layer-7 negotiation for you. Instead, you just send an EOF at the point where you want protocol negotiation to start. With SMTP and IMAP both, this is so markedly better than the alternatives that there's little basis for comparison.
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