Magento – Magento 2.3: How to solve the message “Unable to send email”

I have a problem with Magento 2.3 on the contact page and also on admin with email not been send.

I'm getting this error on the contact page " “Unable to send email”.

I have flow up email extension that when i try to go into the email I get this errors:

2 exception(s): Exception #0 (ReflectionException): Class
Magento\Email\Model\Source\Variables does not exist Exception #1
(ReflectionException): Class Magento\Email\Model\Source\Variables does
not exist

Also on confirmation order that been send out it shows FROM: decornmo@ca-apro2.aspirationcloud.com and not FROM: sales@babybeddingdesign.com

Preconditions:
A Magento 2.3.x instance with at least one product. Reproduced on 2.3.x
Steps to reproduce:

1. I tried update Magento with recently get the recent update and ran
   the commands to set up upgrade, di:compile, deploy static contents,
   clear cache, etc to check the issue. It could not solve the error. I
   checked and found that many of them reported the mail issue with
   Magento 2.3.0. In there, I can see that some class removed from
   latest Magento(2.3.0) which was included in the older version. Also,
   it generate compatibility issue with SMTP module. Please see the URL
   below for more details. Also, I noticed some compatibility issue
   mentioned with Magento 2.3.0, Porto theme and MagePlaza extension. I
   am sure about this and how to solve the error as well.
   mageplaza/magento-2-smtp#127
2. after reading this:
   https://stackoverflow.com/questions/52054370/magento-2-email-sender-incorrect
   I've disabled Amazon plugin and deployed static file. This not
   affect anything.
3. I've uploaded old jquery version to v3 from v1. Magento2.3.0 core
   not using latest jquery version.