[exim] 15s delay after SMTP authentication

Hi all,

I have a problem after upgrading to Debian Bullseye (Exim 4.94.2):

Sometimes (appr. every 3rd try) an outgoing Email takes about 15sec.

I have turned on debugging in config-file with:
MAIN_LOG_SELECTOR == MAIN_LOG_SELECTOR +all


But in the logs I can just see this:
---------------------------------
2021-12-23 08:57:27.464 [196843] SMTP connection from
[5.45.110.153]:39612 I=[5.45.110.153]:587 (TCP/IP connection count = 1)
2021-12-23 08:57:42.731 [196847] 1n0Iyd-000pCx-J3 <=
fellows@??? H=mx.fairtragen.de (thunderleon)
[5.45.110.153]:39612 I=[5.45.110.153]:587 P=esmtpsa L.-
X=TLS1.3:ECDHE_X25519__RSA_PSS_RSAE_SHA256__AES_256_GCM:256 CV=no
A=plain_server:fellows@??? S=563 M8S=0 RT=0.001s
id=20211223085727.196846@??? T="test Thu, 23 Dec 2021
08:57:27 +0100" from <fellows@???> for leon@???
2021-12-23 08:57:42.737 [196847] SMTP connection from mx.fairtragen.de
(thunderleon) [5.45.110.153]:39612 I=[5.45.110.153]:587 closed by QUIT
2021-12-23 08:57:42.756 [196852] cwd=/var/spool/exim4 3 args:
/usr/sbin/exim4 -Mc 1n0Iyd-000pCx-J3
2021-12-23 08:57:42.767 [196852] 1n0Iyd-000pCx-J3 => leon
<leon@???> F=<fellows@???> P=<fellows@???>
R=my_domains T=virtual_mailboxes S=679 QT=15s DT=0.006s
2021-12-23 08:57:42.768 [196852] 1n0Iyd-000pCx-J3 Completed QT=15s
---------------------------------

So there is the SMTP connection at 08:57:27
then 15s nothing (!?)
and then at 08:57:42 the Email processing starts.

What could I do to better debug this?

I am sending the test-email with swaks. So there is no problem with
Thunderbird or the internet connection.
I have also switched off the clamav-checking -> no change.


Thanks!
Leon