Sending more than one email raise SMTPConnectError

[tauhuang]

Version: 0.11.214 ENV: Python3.6

The code below will raise SMTPConnectError: (421, b'Too many connections')

for i in range(5):
    yag.send(to='***', subject='test', contents='Good afternoon')

The code below work well

for i in range(5):
    self.close()
    yag.send(to='***', subject='test', contents='Good afternoon')

Pull Request: https://github.com/kootenpv/yagmail/pull/117