mails queued while server is sending are not sent inmediately

[adbenitez]

Describe the bug
If the server is sending the list of queued mails and in the mean time new mails are queued, the server doesn't check if new mails were queued and instead thinks the work is done and schedules a re-check in 10 minutes.

To Reproduce
Steps to reproduce the behavior:

1. send one email while the server is in the process of sending other emails
2. the messages queued while other were sending are not sent immediately

Log

[  Yggmail  ] Incoming SMTP session from 4c5c81b8379f58ab0f3af9ed186988fb2a05a7c0c90829283e8bb9f0e3438058
[  Yggmail  ] Stored new mail from 4c5c81b8379f58ab0f3af9ed186988fb2a05a7c0c90829283e8bb9f0e3438058@yggmail
[  Yggmail  ] Authenticated IMAP user from 127.0.0.1:50046 as "439a18f3692be6cc3bdf724994d6027a1dc457ef8adf33d68564205c03b3ad46"
[  Yggmail  ] Authenticated IMAP user from 127.0.0.1:50048 as "439a18f3692be6cc3bdf724994d6027a1dc457ef8adf33d68564205c03b3ad46"
[  Yggmail  ] Queued mail for [4c5c81b8379f58ab0f3af9ed186988fb2a05a7c0c90829283e8bb9f0e3438058@yggmail]
[  Yggmail  ] There are 1 mail(s) queued for 4c5c81b8379f58ab0f3af9ed186988fb2a05a7c0c90829283e8bb9f0e3438058
[  Yggmail  ] Sending mail from 439a18f3692be6cc3bdf724994d6027a1dc457ef8adf33d68564205c03b3ad46@yggmail to 4c5c81b8379f58ab0f3af9ed186988fb2a05a7c0c90829283e8bb9f0e3438058
// HERE A SECOND EMAIL WAS QUEUED:
[  Yggmail  ] Queued mail for [4c5c81b8379f58ab0f3af9ed186988fb2a05a7c0c90829283e8bb9f0e3438058@yggmail]
// HERE THE FIRST MAIL WAS SENT BUT THE SERVER DON'T CHECK IF THERE IS SOMETHING NEW IN THE QUEUE:
[  Yggmail  ] Sent mail from 439a18f3692be6cc3bdf724994d6027a1dc457ef8adf33d68564205c03b3ad46@yggmail to 4c5c81b8379f58ab0f3af9ed186988fb2a05a7c0c90829283e8bb9f0e3438058
// ONLY ONE EMAIL SENT, THE SECOND QUEUED EMAIL WAS NOT SENT

Additional context
I realized this while trying a Delta Chat "Echo Bot", that quickly sends one mail with read receipt and another echoing back the text, since this happens too fast, always the second message is queue while the fist one is sending so the second message gets delayed by 10 minutes, or until another message arrives.