Alertmanager receiver for Mattermost not working

Followed the instructions for setting up the Alertmanager receiver for Mattermost and have an incoming webhook set up. However, I get the following error messages in the alertmanager pod:

ts=2023-05-21T23:09:43.021Z caller=notify.go:732 level=warn component=dispatcher receiver=mattermost integration=slack[0] msg="Notify attempt failed, will retry later" attempts=10 err="Post \"<redacted>\": read tcp 10.42.4.117:42616-><redacted>:443>: read: connection reset by peer"
ts=2023-05-21T23:09:43.021Z caller=notify.go:732 level=warn component=dispatcher receiver=mattermost integration=slack[0] msg="Notify attempt failed, will retry later" attempts=1 err="Post \"<redacted>\": read tcp 10.42.4.117:42604-><redacted>:443: read: connection reset by peer"
ts=2023-05-21T23:09:45.946Z caller=notify.go:732 level=warn component=dispatcher receiver=mattermost integration=slack[0] msg="Notify attempt failed, will retry later" attempts=10 err="Post \"<redacted>\": read tcp 10.42.4.117:54786-><redacted>:443: read: connection reset by peer"
ts=2023-05-21T23:09:45.946Z caller=notify.go:732 level=warn component=dispatcher receiver=mattermost integration=slack[0] msg="Notify attempt failed, will retry later" attempts=10 err="Post \"<redacted>\": read tcp 10.42.4.117:54778-><redacted>:443: read: connection reset by peer"
ts=2023-05-21T23:09:48.590Z caller=notify.go:732 level=warn component=dispatcher receiver=mattermost integration=slack[0] msg="Notify attempt failed, will retry later" attempts=10 err="Post \"<redacted>\": net/http: TLS handshake timeout"

Does alertmanager use a specific account to post to mattermost. How does it authenticate to be able to post to the public URL to mattermost?