[exim] require verify = sender ACL

In my acl_smtp_rcpt

I have a:

require verify = sender

which 'usually' lets a particular sender get to me, but occasionally
fails that sender with a:
2014-12-31 13:50:14 H=air.registry.ca [192.75.213.8] sender verify fail
for <nouser@???>: all relevant MX records point to non-existent
hosts

I attribute this to failed DNS. At least, that's what I presume.
So research shows that I could switch to "try verify" rather than
"verify"
I cannot find any examples with this acl.

I am unsure of the syntax of this. Will this still work within the
acl_smtp_rcpt.

Thanks for suggestions.

Jim