[exim] Host format in route_data

I have some trouble with specifying a fixed smtp host in the route_data
configuration item for a manualroute router, intended for the smarthost
usage pattern.

I have two servers, both with identical exim.conf files. exim.conf
contains:

smarthost:
driver = manualroute
domains = ! +local_domains
transport = remote_smtp
route_data = SMARTHOST
ignore_target_hosts = <; 0.0.0.0 ; 127.0.0.0/8
no_more

On server A running exim 4.87, SMARTHOST is defined thus:

SMARTHOST = 10.8.78.1

and this seems to work fine.

Now I want to do the same on server B, running exim 4.84, but with

SMARTHOST = 127.0.25.25::2525

but this unfortunately results in logs like the following:

2016-08-05 09:59:57 H=(sending.host.com) [10.8.78.14]
sender verify defer for <itz@???>:
lookup of host "127.0.25.25" failed in smarthost router
2016-08-05 09:59:57 H=(sending.host.com) [10.8.78.14]
X=TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256 F=<itz@???>
temporarily rejected RCPT <me@???>: Could not complete sender verify

Neither 10.8.78.1 nor 127.0.25.25 are listed in the respective /etc/hosts
file, and no other non-DNS name lookup (like NIS) is enabled, if that matters.

I also tried enclosing the address in brackets:

SMARTHOST = [127.0.25.25]::2525

but with the same result.

What gives?

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