I've managed to get the desired result with a patch below:

--- a/src/mail/ngx_mail_proxy_module.c
+++ b/src/mail/ngx_mail_proxy_module.c
@@ -574,7 +574,7 @@

cscf = ngx_mail_get_module_srv_conf(s, ngx_mail_core_module);

- line.len = sizeof("HELO ") - 1 + cscf->server_name.len + 2;
+ line.len = sizeof("HELO ") - 1 + s->smtp_helo.len + 2;
line.data = ngx_pnalloc(c->pool, line.len);
if (line.data == NULL) {
ngx_mail_proxy_internal_server_error(s);
@@ -587,7 +587,7 @@
((s->esmtp || pcf->xclient) ? "EHLO " : "HELO "),
sizeof("HELO ") - 1);

- p = ngx_cpymem(p, cscf->server_name.data, cscf->server_name.len);
+ p = ngx_cpymem(p, s->smtp_helo.data, s->smtp_helo.len);
*p++ = CR; *p = LF;

if (pcf->xclient) {