Using SMTPLIB with Python3 - Getting (501) "Syntax error - line too long" Error When trying to send the email

Here is the syntax I use to build an email, the email itself will send unless I add an attachment, then I get the error.

Or if my HTML is too long, I'll get the error:

# MSG building
msg = MIMEMultipart("alternative")
msg.attach(MIMEText(email_text, "plain"))
msg.attach(MIMEText(email_html, "html"))

msg["To"] = to_email
msg["From"] = from_email
msg["Subject"] = subject
msg["Date"] = formatdate()
msg["Message-ID"] = make_msgid(domain=sender_domain)

with open(attachment_dir, 'rb') as f:
    part = MIMEBase("application", "octet-stream")
    part.set_payload(f.read())
    
    # Encode to Base64                
    encoders.encode_base64(part)
    
    # Set mail headers
    part.add_header(
        "Content-Disposition", "attachment", filename=attachment_name
        )
    
    msg.attach(part)
    
# Python 3 libraries expect bytes.
msg_data = msg.as_bytes()

# Create secure connection with server and send email
context = ssl.create_default_context()

with smtplib.SMTP_SSL("smtp.ionos.co.uk", 465, context=context) as mailserver:
    mailserver.login(from_email, from_password)
    mailserver.sendmail(from_email, to_email, msg_data)

Here is the Traceback I'm getting (It's a standard line too long error):

Traceback (most recent call last):
  File "/home/server/repos/mailsender/run.py", line 33, in <module>
    email.send()
  File "/home/server/repos/mailsender/main.py", line 139, in send
    mailserver.sendmail(from_email, to_email, msg_data)
  File "/usr/lib/python3.9/smtplib.py", line 892, in sendmail
    raise SMTPDataError(code, resp)
smtplib.SMTPDataError: (501, b'Syntax error - line too long')

Syntactically, how do I avoid this- I understand the error message, but how can I get around that and fix it in Python3

Thanks in advance

Here is the Image I tried to attach: https://i.stack.imgur.com/QTprF.jpg

It's not even 1MB in size.