This was already asked so many times here in the forum…
Simple answer: Create a mailbox for sending such mails via mailcow, and configure the smarthost settings on your web server to include the username/mail address and password for that mailbox. Use port 587 of mailcow
Next level: add your web server as forwarding host in the mailcow options. Use port 25 of mailcow
If needed add the IP to the Postscreen whitelist
But if you are sending to external addresses outside mailcow still DKIM is missing and that needs additional tweaking. So I recommend using the simple answer 😉