VT Final Accounts never generates malformed XML.
VT have observed a few incidences where email programs modify iXBRL files when they are received as attachments. To be more precise, a comment line is added to the beginning of the file before the XML declaration.
Some XML parsing programs will ignore this but HMRC's validator does not. Submissions are rejected with a Malformed XML message.
The workaround is to convert the file to a zip file using Winzip or a similar utility. After the file is received, it needs to be unzipped by the recipient. This prevents the file being modified by the receiving email program.
In VT's experience, it is rare for an email program to modify an iXBRL file on receipt. Hence iXBRL files should normally be emailed as ordinary attachments.