[exim] Increasing smtp_accept_max for submission

Recently I've noticed a few times that I've been unable to send mail
(from my MUA) because the mail server has too many concurrent SMTP conn
ections.

This doesn't work, because it's not expanded:

smtp_accept_max = ${if eq{$interface_port}{587} {25}{20}}

Could it be? Any alternative suggestions?

--
dwmw2