The new Symfony Mailer was introduced in Symfony 4.3. In Symfony 4.4 we improved and polished it, including the integration with the profiler. The debug toolbar now displays the number of messages sent. If you click on it, you'll see the full message details in the new profiler panel:

When making too many Ajax requests or keeping a page open for too long, the debug toolbar gets crowded with a long list of Ajax requests. In Symfony 4.4, you can click on the new Clear link to delete all the existing Ajax requests so you can see the new requests more clearly:

Auto-Select the Best Panel for Ajax Requests¶

When you click on any element of the debug toolbar, you are redirected to its related panel (request, forms, logs, database, etc.) However, for Ajax requests we only display a link to its profile, so you cannot select the first panel displayed.

In Symfony 4.4, we made this link smarter to redirect to the most important panel for each request. For example, if the Ajax request resulted in an exception, you are redirected to the exception panel; if you dumped some information in the Ajax request, you are redirected to the dump panel; etc.

It's a small change, but it will save you some clicks and will improve your developer experience.