Major Description
If you use "fetch 1 body.peek[header.fields (nonexistheader)]" an rfc3501 IMAPServer must respond with a literal of size 2 and writing a blank line.
See rfc3501:
The subset returned by HEADER.FIELDS
contains only those header fields with a field-name that
matches one of the names in the list; similarly, the subset
returned by HEADER.FIELDS.NOT contains only the header fields
with a non-matching field-name. The field-matching is
case-insensitive but otherwise exact. Subsetting does not
exclude the [RFC-2822] delimiting blank line between the header
and the body; the blank line is included in all header fetches,
except in the case of a message which has no body and no blank
line.