problem:

When sending messages from the webmailing account an error message occurs: "There was an error sending your message: Validation failed for: pop<YourIdentifier>@".SYNTAX-ERROR."

rectification:

When using Webmail you have to provide your identity data once for each POP3 account.

Do this as follows:

1. Log in to Webmail.
2. Go to "Options" -> "Personal information"
3. "Standardidentität" [standard identity] is preselected
4. Click on "Edit your identities"
5. Choose the entry "Standardidentität" from the drop-down menu.
6. Provide the following details:
   -> Your From: address (Your sender-eMail)
   -> Your full name: (Your name)
7. Click on the "Change" button to apply the settings.
8. From now on you can send eMails via webmail.