Skipping login with Yagmail gives YagInvalidEmailAddress

Question

I'm trying to test yagmail on my local SMTP server. However, using yagmail.SMTP with smtp_skip_login flag set to True raises as shown below:

import yagmail

with yagmail.SMTP(host='localhost', port=1026, smtp_skip_login=True) as server:
    ...

This raises the following error:

Traceback (most recent call last):
  File "/home/leonarduschen/src/test/main.py", line 3, in <module>
    with yagmail.SMTP(host='localhost', port=1026, smtp_skip_login=True) as server:
  File "/home/leonarduschen/src/test/venv/lib/python3.9/site-packages/yagmail/sender.py", line 51, in __init__
    validate_email_with_regex(self.user)
  File "/home/leonarduschen/src/test/venv/lib/python3.9/site-packages/yagmail/validate.py", line 110, in validate_email_with_regex
    raise YagInvalidEmailAddress(emsg)
yagmail.error.YagInvalidEmailAddress: Emailaddress "@gmail.com" is not valid according to RFC 2822 standards

What did I do wrong?