[Exim] sending messages from queue one-by-one

Hello,

Using exim-4.34 I want to deliver all messages to remote servers one
by one, just one message simultaneously. For this purpose I use the
following:

remote_max_parallel = 1
queue_run_max = 1
queue_run_in_order = true

It works fine when message goes through immediately from the first
attempt but sometimes after defer for example and using 'sendmail -qf'
it seems is does not work correctly and more that one simultaneous
sessions established

Is this right behavour or I have to add something else to config?
Thanks.

--
Best regards,
Sergey