The automatic E-mails to my Customers sometimes end up in their Spam Folder. Can this be prevented?

You can configure your own email server in the menu "Company > E-Mail Sever". All emails are then sent directly from your configured server. This reduces the likelihood of spam classification.

You can get the data you need to fill in the fields from your system administrator or the e-mail provider.
If you use Gmail / Googlemail, you can find instructions here.
If you use Microsoft Office 365, you can find instructions here.

Background information: By default, all emails are sent from the eTermin email server using the address you have stored in Company > Settings > Communication > Sender email address. However, many email clients recognize that the email was sent from the eTermin server and not from the server of the email address. Therefore, the email client then marks it as spam or does not deliver the email.

An alternative to your own email server is to create an SPF record. More information can be found here.