Compatibility with Symfony Mailer

I do not know about Symfony Mailer specifically but this module is fully compatible with the latest Drupal 9.x

Symfony mailer is a replacement for swiftmailer, which is EOL. Swiftmailer was the most common way to send HTML emails from
Drupal. The Drupal symfony_mailer module is therefore experiencing very rapid adoption. For example, it is recommended by Drupal commerce: https://www.centarro.io/blog/replace-swift-mailer-symfony-mailer-html-email

Therefore I believe this module - if it aims to be the sensible normal option for integrating Sendgrid with Drupal - should support Symfony mailer. This will likely involve a plugin for drupal/symfony_mailer; it may also involve a custom symfony mailer transport, but we may be able to avoid that and depend on the symfony/sendgrid-mailer package.

Regarding symfony mailer and sendgrid, I found this informative:
https://symfonycasts.com/screencast/mailer/sendgrid

YES! Just spent several days trying to figure out issues sending email from a new D9 site and finally ended up here hoping there was a solution for the sg_integration dependency on swift when symfony seems to be the next gen for drupal HTMK emails. Thanks so much for all the great work on the module to date. We've used it for several clients and has been great. Would love to keep using it in the future.

You don't need sendgrid_integration module to use Sendgrid with Symfony Mailer module.

Do this:
1. Uninstall sendgrid_integration and mailsystem modules.
2. Add symfony/sendgrid-mailer package with composer.
3. Install symfony_mailer module.
4. On the Transport tab, add a DSN one, and with this config: sendgrid://KEY@default. KEY is your Sendgrid key. Save.

Haris Khan Jadoon → made their first commit to this issue’s fork.

Thanks, @rpayanm!
OMG, so easy but little documentation at this point. I'm going to add some keywords to the thread to make it easier for other people to find...
Drupal 10 Symfony Mailer not working with Sendgrid API.
Common errors:

• drupal symfony mailer Unable to write bytes on the wire
• drupal Error sending email: Unsupported sendmail command flags "/bin/true"; must be one of "-bs" or "-t" but can include additional flags.

one more thing to add to @rpayanm's list was to be sure to set the new DSN transport to default. Mine was the "Sendmail" one that was there when I installed the module. I didn't realize I had to set the default.

I can confirm that @rpayanm's suggestion in #5 is a working solution. Thanks for the tip Rolando!