Complex sentence: Non-standard header fields were conventionally marked by prefixing the field name with X- but this convention was deprecated in June 2012 due to the inconveniences it caused when non-standard fields became standard.
Split into simpler sentences: Non-standard header fields were conventionally marked by prefixing the field name with X-. However, this convention became deprecated in June 2012 due to the inconveniences it caused when non-standard fields became standard.