Cannot send out mails

In my status check the following is listed:

Outbound mail (SMTP port 25) seems to be blocked by your network. You will not be able to send any mail. Many residential networks block port 25 to prevent hijacked machines from being able to send spam. A quick connection test to Google’s mail server on port 25 failed.

Is there any chance to solve this; i.e.other port number, but where to configure in the box?

Generally the solution is to ask those who run the network to unblock that port.
For example, on Linode, you first make sure that everything else (for example rDNS) is configured properly, then you create a support ticket to unblock the port (where you also confirm that your email practices conform to CAN-SPAM act)

Thank you for th e quick answer.
But I am only a simple user and I don’t know who on the way block outgoing mails.
a) my Inetrnet provider
b) my DigitalOcean Droplet with Ubuntu 18.04 (firewall allow 25 tcp)
c) MiaB

Can I change the port somewhere?

a) Internet provider is an incredibly generic term. It can mean your wireless phone provider, residential internet provider, hosting company, etc. So I’m not clear what is being asked.

b) If Digital Ocean is your hosting provider, then this is the most likely cause of problems related to ports. Note that assuming in the command line installation of MiaB the server provided confirmation of a successful installation, it is highly unlikely to be a problem related to Ubuntu 18.04.

c) MiaB is intended to provide complete auto-configuration of the server, including the server’s firewall configuration. Based on the message reported in the GUI system checks, it is unlikely that MiaB is the cause of this problem.

It is generally not recommended to make custom changes to the ports unless you are an expert and intend to maintain your own unsupported customizations of MiaB.

On new Digital Ocean accounts you must specifically request that email ports be unblocked by opening a support ticket. This has nothing to do with any firewall settings.