First you should check that your Ubuntu machine’s postfix is configured to send encrypted mail on port 587 with STARTTLS to Mailcow. You also need to authenticate with Mailcow using a good username and password.
A little off topic (just inb case it is useful), I use sSMTP instead of Postfix in the simple case where I am forwarding logs and notifications. This is easy to set up, and may be a good way for you to test the relaying. A working sSMTP configuration with Mailcow looks like this:
# Config file for sSMTP sendmail
# The person who gets all mail for userids < 1000
# Make this empty to disable rewriting.
# The place where the mail goes. The actual machine name is required no
# MX records are consulted. Commonly mailhosts are named mail.domain.com
# Where will the mail seem to come from?
# The full hostname
# Are users allowed to set their own From: address?
# YES - Allow the user to specify their own From: address
# NO - Use the system generated From: address