Allow multiple EHLO/HELO commands as per the RFC

Fixes:

````
 ----- Transcript of session follows -----
... while talking to mail1-ec2.fsl.com<http://mail1-ec2.fsl.com>.:
MAIL From:<xxx@xxx> SIZE=nnnn
BODY=7BIT
<<< 503 You already said HELO
554 5.0.0 Service unavailable
````