For all eml in a folder, append an identifier to the subject line (if it doesn't already exist) and add a subject line if it was missing.

add_prefix_to_eml_subjects.py

#!/usr/bin/env python3
import os
import sys

def process_eml(file_path, subj_prefix):
    try:
        with open(file_path, 'r', encoding='utf-8', errors='ignore') as f:
            lines = f.readlines()

        modified = False
        subject_found = False

        for i, line in enumerate(lines):
            if line.lower().startswith("subject:"):
                subject_found = True
                subject_line = line.strip()
                # Add prefix only if not already present
                if not subject_line.startswith(f"Subject: {subj_prefix}"):
                    new_subject = f"Subject: {subj_prefix}{subject_line[len('Subject: '):]}"
                    lines[i] = new_subject + "\n"
                    modified = True
                break  # only process the first Subject line

        if not subject_found:
            # Insert new Subject line only once
            new_subject_line = f"Subject: {subj_prefix}\n"
            # Optional: insert before the first blank line or header boundary
            insert_index = 0
            for j, line in enumerate(lines):
                if line.strip() == "":
                    insert_index = j
                    break
            lines.insert(insert_index, new_subject_line)
            modified = True

        if modified:
            with open(file_path, 'w', encoding='utf-8') as f:
                f.writelines(lines)
            print(f"Updated: {file_path}")
        else:
            print(f"No change: {file_path}")

    except Exception as e:
        print(f"Error processing {file_path}: {e}")

def main():
    if len(sys.argv) < 3:
        print("Usage: add_prefix_to_eml_subjects.py <folder_path> <subj_prefix>")
        sys.exit(1)

    folder = sys.argv[1]
    subj_prefix = sys.argv[2]

    for root, _, files in os.walk(folder):
        for file in files:
            if file.lower().endswith(".eml"):
                process_eml(os.path.join(root, file), subj_prefix)

if __name__ == "__main__":
    main()

Usage:

% python3 add_prefix_to_eml_subjects.py path/to/folder/of/eml "[prefix] " 

(Note the trailing space after the closing bracket ])