Trouble with mail on 9.706.9 - too many connections

Since Updating our SG550 to 9.706.9 we have trouble with mail. E-Mails are delayed or even not delivered at all.  

E-Mail senders get the following message:

Remote Server returned '554 5.4.0 < #4.0.0 X-Proxmox; delivery temporarily suspended: host x.x.x.x[x.x.x.x] refused to talk to me: 421 Too many concurrent SMTP connections; please try again later.>'

or: 421.4.4.2 connection dropped due to TimeOut

I also recognize high cpu load (100%) on exim processes

In smtp log I often see tcp connection count 25, so all allowed connection are beeing used.

When increasing the allowed connections whole utm perfomance goes down and httpproxy will be very sloooow .

Max connection: 25 (default: 20)

Max connections / host: 15 (default: 10)

Any ideas for me?

Markus