POST SMTP Sends Email but Site Times out

When creating a new website instance on multisite network or trying to send email, I am having a strange problem: the email gets send but the site returns HTTP Error 503. The service is unavailable. The server is temporarily unable to service your request due to maintenance downtime or capacity problems.

Wen I disable your plugin and add the following snippet the form to create a new website works and I do not get any HTTP errors:

function prefix_disable_mails( $phpmailer ) {
    $phpmailer->ClearAllRecipients();
}
add_action( 'phpmailer_init', 'prefix_disable_mails' );

However, in this way, I am not sending any emails.

When I checked server logs, I reailised the new instance is sending mails, that is why I disabled sending mails and the HTTP ERROR is gone but this is clearly not working for me, since I want that the user gets an email when a new website is created and he is the admin.

Any help will be appreciated, I realise the issue is most probably with the host but unfortunately I am stuck with it. Perhaps you have experienced such problem in the past?

Sendgrid is properly configured and so is your plugin, however I still get this capacity problem, even though i have increased the memory limit to 2048MB