Unable to send emails via mailgun after upgrade

Describe the issue or problem
I have been using Mailgun for sending emails through OJS for the past several years. I recently sifted to DigitalOcean and upgraded to OJS 3.4 from 3.3. Following the upgrade, I have not been able to send any emails.

Steps I took to solve this issue

  1. I have already installed the Mailgun layer from Vitaliy-1. However, that did not solve the issue.
  2. I have tried different combinations of configurations in the config file.
  3. I believe I cannot use send mail because DigitalOcean blocks port 25 by default.

What application are you using?
For example, OJS 3.4.0-5

Additional information
Here is output from tail -f /var/log/apache2/ojs.error.log
[Sun Mar 10 04:51:28.130432 2024] [php:notice] [pid 2827] [client] Failed to authenticate on SMTP server with username “postmaster@mail.mydomain.org” using the following authenticators: “LOGIN”, “PLAIN”. Authenticator “LOGIN” returned “Expected response code “235” but got code “535”, with message “535 Authentication failed”.”. Authenticator “PLAIN” returned “Expected response code “235” but got empty code.”., referer: https://mydomain.org/index.php/chem/workflow/index/297/5