submission proxy -- where to config/present client cert?

 I've a dovecot instance setup with submission proxy,

protocols = imap lmtp submission sieve
hostname = internal.mx.example.com
submission_relay_host = internal.mx.example.com

submission_relay_port = 465
submission_relay_trusted = yes
submission_relay_ssl = smtps
submission_relay_ssl_verify = yes

service submission-login {
	inet_listener submission {
		address = 10.2.2.10, 127.0.0.1
		port = 50465
		ssl = yes
	}
}

protocol submission {
	ssl_cert = < /sec/vmail/mx.example.com.server.EC.crt.pem
	ssl_key  = < /sec/vmail/mx.example.com.server.EC.key.pem
	ssl_verify_client_cert  = no
}

It submits to a postfix instance, atm on the same host,

[internal.mx.example.com]:465 inet n - n - - smtpd
 -o smtpd_tls_req_ccert=no

with that setup, all works as expected.

If I turn ON required client cert verification @ postfix,

• -o smtpd_tls_req_ccert=no

• -o smtpd_tls_req_ccert=yes

with that add'l req't, postfix log reports "no client certificate presented"

Jul 18 14:52:38 mx postfix/submit-from-dovecot/smtpd[37133]: connect from internal.mx.example.com[10.2.2.10]
Jul 18 14:52:39 mx postfix/submit-from-dovecot/smtpd[37133]: Anonymous TLS connection established from internal.mx.example.com[10.2.2.10]: TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-384) server-digest SHA384
Jul 18 14:52:39 mx postfix/submit-from-dovecot/smtpd[37133]: NOQUEUE: abort: TLS from internal.mx.example.com[10.2.2.10]: No client certificate presented
Jul 18 14:53:09 mx postfix/submit-from-dovecot/smtpd[37133]: disconnect from internal.mx.example.com[10.2.2.10] commands=0/0

Clearly, I'm missing config.

What in my dovecot submission config^^ do I need to change/add to get it to 'present' a client cert to the postfix relay for client cert verification?