How to send mail from drupal 7 efficently and avoid 'Too many concurrent SMTP connections' error? I'm using the smtp module to send mail from a drupal 7 installation throught gmail. In

 /admin/config/system/smtp


I set 

 smtp server 'smtp.gmail.com' 
 smtp port '587' 
 use encryption 'use tls' 


and i set my valid mail 'something@gmail.com'.

Now, it's working, but some days ago a lot of mail, on the account something@gmail.com, began to arrive containing an error message:

 Delivery to the following recipient has been delayed:
 someBadGuy@somemail.com
 Message will be retried for 2 more day(s)
 Technical details of temporary failure:
 The recipient server did not accept our requests to connect. Learn more at https://support.google.com/mail/answer/7720
 [(10) somemail.com. [someip]:25: 421 Too many concurrent SMTP connections; please try again later. ]


There were and there will always be on my drupal site people registering with fake unreachable mail, so assuming i can't solve this problem, what this message means? It began some days ago, out of the blue, so i think something is changed, but i do not know precisely what. And do not know precisely where. (gmail, my provider, my server...) 

Anyway, what is the best way to send mail from drupal, excluding the idea of setting up my own mail server and excluding gmail, as it implies this problem?
Have you tried using the Mandrill module? https://www.drupal.org/project/mandrill