Slow mail deliveries from the internet

We are using the Open Xchange app since several Months now. We are happy to have a stable and proven e-mail system with a minimal amount of administration effort.

Sometimes our other systems have to provide a token to get things initiated. Those tokens are valid for a certain time. Well, if the mail arrives more than 10 Minutes late we need a now one. Then two tokens arrive which will mess up processes and this will generate a lot of support effort.

Because of that I performed several tests with a mail server within our country, from another European states and from a US company. I just got one attempt taking 16 Minutes. The others operated well and got delivered within Minutes.
My colleagues still have more hassle than I had.

I tried to investigate this even I have quite no expertise regarding email mechanics and tuning. In the end I found that even fetchmail is in use but systemctl is not on top of it but seems utilised within the UCS setup. Beside the question how ever could fetchmail help when we get messages from a random outside mail server, I did not find a UCS process, service or registry entry about.
I also checked that all involved protection measures like clamAV and SpamAssassin are using valid, fresh data.

Is there any idea how I could improve the delivery time?