VDS mail users which are created with ftp disabled , are created with no home directory. This would fail any arriving mail for that user.
This is a known bug in SD 3.7
Two Workarounds for this situation:
1. Edit VDS/etc/passwd manually to add home directory to the mail user.
If "turtle" is the user :
should be changed to:
2. Create the user with ftp enabled.
That would create a home directory for the user.
You can disable FTP for the specific user afterwards.
Problem is fixed in UP1 for SD 3.7