Some actions will trigger a new email notification that will be sent according to your settings.
You can change the options in
Options -> E-mail notifications
Here you can specify an email to send from, add BCC copies to other emails, set a maxmium retries attempt for each notification in case they fail, and customize your sender/SMTP settings.
A very important setting can be found under the System performance group, with the name Send "New file" email notifications during the file saving process.
This will prevent ProjectSend to send emails as soon as files are assigned, making the editing process much faster. Be aware that if you enable this setting, it's recommeded that you also set up a cron task that takes care of sending these emails or neither users or clients will be notified of any new files.