CMSMailer 1.73.13 test message and also registration at Forum Made Simple using
SelfRegistration causes issues if the mail server authorization is incorrect or
the mail server is either unavailable or else rejects the message. In the case
of registration, it does silently succeed.

In the web browser, an attempt is made to redirect, but the page carries on
loading and loading. The default SMTP time-out is 1000 seconds for some strange
reason, so it's possible that the process might time out, though I haven't
tested this.

At the Apache server, other PHP pages become unresponsive and though the
server's windows service can be stopped and restarted, stopping takes quite a
few seconds, so it's clearly busy, though just consuming the normal percentage
of CPU. It doesn't hang any other applications, nor does it crash Windows
Explorer.

I presume that rather than send the message asynchronously and redirect
immediately, the system actually waits until the message has been sent ... which
doesn't happen.