Hello,
I wanted to set up a small temporary mail server, but apparently my ISP has blocked the default port 25.
Does Cloudflare offer a way to redirect port 25 to another port or collect the mails and redirect them to another port?
Thank you for your support!
Welcome to the Cloudflare Community. 
The short answer is: no. SMTP is not a port that Cloudflare supports.
Your diagram appears to be referencing inbound SMTP. Although your objective it is not clear, a better option would be to host your edge MTA on a VPS, and, if your goal is to run an MDA locally, you can simply use the local device to fetch the mail from your public host. Outbound SMTP would need to be smarthosted. The submission port on 587 with STARTTLS or the submissions port of 465 with implicit TLS should be accessible.
The specifics of how to implement all of this are out of scope for discussion in the Cloudflare Community.
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