[exim] Some mails are queued, other are sent immediately

Hi,

I am new to Exim. I use it so that Debian servers can send me
notifications (through a smarthost).

First, maybe I should use a simpler MTA but I need :
- queueing (it can happen that the server has not Internet connection
and I do not want to lost any notification)
- ability to rewrite subject (this help recognize which server sent the
mail without having to look the email headers)
- ability to use a smarthost (and thus ability to rewrite "From:" header)

Anyway, during tests I noticed that some emails are sent immediately
(what I prefer), while other are queued (introducing useless delay):
- Why this difference ?
- What could I do to avoid the queue ?

Here is a sample log showing a two emails behaving differently:

2017-12-16 11:18:05 Start queue run: pid=20982
2017-12-16 11:18:05 End queue run: pid=20982
2017-12-16 11:21:30 1eQ9b0-0005YG-C1 <= yvan@???
U=root P=local S=452
2017-12-16 11:21:35 1eQ9b0-0005YG-C1 => contact@???
R=smarthost T=remote_smtp_smarthost H=smtp.phpnet.org [194.110.192.60]
X=TLS1.2:RSA_AES_256_CBC_SHA1:256 CV=yes DN="CN=*.phpnet.org" A=plain
C="250 OK id=1eQ9b5-0000dU-4e"
2017-12-16 11:21:35 1eQ9b0-0005YG-C1 Completed
2017-12-16 11:34:37 1eQ9nh-0005zG-PL <= yvan@???
U=root P=local S=180280
2017-12-16 11:48:05 Start queue run: pid=24317
2017-12-16 11:48:11 1eQ9nh-0005zG-PL => contact@???
R=smarthost T=remote_smtp_smarthost H=smtp.phpnet.org [46.255.160.212]
X=TLS1.2:RSA_AES_256_CBC_SHA1:256 CV=yes DN="CN=*.phpnet.org" A=plain
C="250 OK id=1eQA0o-0006sY-4W"
2017-12-16 11:48:11 1eQ9nh-0005zG-PL Completed
2017-12-16 11:48:11 End queue run: pid=24317


I have read the FAQ, but I could not find something an answer.

Regards,
Yvan