No To: field leads to "RFC compliant TO required" mail server error

Hello everyone.

I have upgraded RT system, with following changes:
FreeBSD 6.2 -> 10.1
RT 3.8.8 -> 4.2.11
Postfix 2.4.6 -> Exim 4.85

On the new system i’m having error in mail system “RFC compliant TO
required” when RT tries to send mails about my comments on mine ticket.
As far as i understand the logic behind, RT compose email with ticket’s
author email in To: header, and all other queue’s followers in Cc:
header, then see that it’s me, the comment’s author, is in To: header,
and configured for not sending me email about my own comment, strips my
email and effectively entire To: header from email.

Then there is a difference between old and new system: on the old system
everything was exactly the same as on the new system (according to email
headers in rt.log), but emails without To: header somehow were delivered
to all Cc: addresses, but on the new system i get this error.

Logs from the old system: (there is “To:” header in the beginning, and then it’s gone)
Logs from the new system: (same with “To:”)

Emails from the old system have To: header with
"undisclosed-recipients:;" in it. Emails from the new system, according
to Mailer-Daemon messages, have not.

I think that maybe the old system’s Postfix mail server inserting To:
undisclosed-recipients:; header. I’ve tried to install and use new
version of Postfix 2.11.6 with no changes - emails still doesn’t delivered.

To overcome this situation i have configured Exim with the following ACL:

warn condition = ${if !def:h_To: {1}}
add_header = To: Undisclosed-Recipients:;

But i wonder, if there is a better for populating To: header with “undisclosed-recipients:;”? Or maybe there is something i missed in upgrade? I did run rt-setup-database after moving DB to new host, if that’s matter.

best regards