Internal helper function for loop execution used in fetch functions.
loop_fetch_msg_header(new_imapconf, msg_id, by, fields, negate_fields,
  peek, partial, write_to_disk, keep_in_mem, retries, handle)A numeric vetor containing one or more messages ids.
Which id (MSN or UID) to use when searching for
messages. "MSN" or message squence number is a message's relative
position to the older message in the mailbox. It may change after deleting
or moving messages. For instance, if a message is deleted, message sequence
numbers are reordered to fill the gap. "UID" or unique identifier
is always the same during the life cycle of a message. Default is
"MSN".
A character vector specifying which field(s) to fetch from messages. See section_or_field_options
If TRUE, negates the search and seeks for
"NOT in the field". Default is FALSE.
If TRUE, it does not mark messages as "read" after
fetching. Default is TRUE.
NULL or a character string with format
"startchar.endchar" indicating the size (in characters) of a message slice
to fetch. Default is NULL, which fetchs the full specified content.
If TRUE writes fetch content of each message
to the disk as a text file in the working directory. Default is FALSE.
If TRUE keeps a copy of fetch results as an
list in the R session when write_to_disk = TRUE. Default is
FALSE. It can only be set TRUE when
write_to_disk = TRUE.
Number of attempts to connect and execute the command. Default
is 2.
A curl handle object.
An object of class imapconf generated with
configure_imap and with a mbox item added after a
mailbox selection with select_mailbox.
A list or/and text files containing the fetch results.
Other fetch helpers: loop_fetch_full_msg,
  loop_fetch_msg_metadata,
  loop_fetch_msg_text
Other loop: loop_fetch_full_msg,
  loop_fetch_msg_metadata,
  loop_fetch_msg_text