flask handle thread exceptions - multithreading

Question

I'm trying to display an error if there is an exception in my asynchronous
send_email function but even after an exception is occurred in the background thread the data still gets stored in the database. The except block never gets executed.
I'm trying to prevent the database commit and redirect to a page with an error message
send_email function file
from threading import Thread
from flask import current_app,render_template,flash
from flask_mail import Message
from feedback import mail
def send_async_email(app, msg):
try:
with app.app_context():
mail.send(msg)
except:
raise Exception("Exception occurred in Mail")
return False
def send_email(to,subject,template,**kwargs):
app = current_app._get_current_object()
msg=Message(current_app.config['MAIL_SUBJECT_PREFIX'] + subject,
sender=current_app.config['MAIL_SENDER'],recipients=[to])
msg.body=render_template(template +'.txt',**kwargs)
msg.html=render_template(template +'.html',**kwargs)
try:
thr = Thread(target=send_async_email, args=[app, msg])
thr.start()
return thr
except:
raise Exception("Error occured")
views.py
#home.route('/register',methods=["GET","POST"])
def register():
# form=LoginForm()
form=LoginForm(prefix="login")
form1=RegisterForm(prefix="register")
print(form1.errors)
if form1.validate_on_submit():
users=Users(email=form1.email.data.strip())
users.set_password(form1.password.data.strip())
organization=Organization(organization_name=form1.org_name.data.strip())
organization.userorg.append(users)
db.session.add(users)
db.session.add(organization)
db.session.flush()
token=users.generate_confirmation_token()
try:
print("INSIDE TRY BLOCK")
email=send_email(users.email,'Confirm your account','email/confirm_email',user=users,token=token)
except Exception as e:
print("INSIDE Exception BLOCK")
print(e)
flash("Errors","danger")
db.session.rollback()
return redirect(url_for('home.home_view'))
db.session.commit()
flash("A confirmation email has been sent")
return redirect(url_for('home.home_view'))
return render_template('home/landing-semi.html',form=form,form1=form1)
Errors recieved
raise SMTPSenderRefused(code, resp, from_addr)
smtplib.SMTPSenderRefused: (530, b'5.5.1 Authentication Required. Learn more at\n5.5.1 https://support.google.com/mail/?p=WantAuthError g10-v6sm7818697pfi.148 - gsmtp', '=?utf-8?q?Osfapp_Admin?= <lmnography#gmail.com>')
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/usr/lib64/python3.6/threading.py", line 916, in _bootstrap_inner
self.run()
File "/usr/lib64/python3.6/threading.py", line 864, in run
self._target(*self._args, **self._kwargs)
File "/home/leon/Leon/Projects/fmsappv/feedback/modules/utilties.py", line 12, in send_async_email
raise Exception("Exception occurred in Mail")
Exception: Exception occurred in Mail