client incorrect greeting error, how to resolve?

[Voytek]

I have a user reporting from time to time getting:
"An error occurred while sending mail. Mail server sent incorrect greeting
4.7.0 geko.sbt.net.au error too many connections from 147.50.1.226"

is this a Thunderbird issue ?

when I search like below, I get nothing, what am I doing wrong?

and, how to fix this, is there anything at this end ? user end ?


# grep 147.50.1.226   /var/log/maillog | wc
    407    7142   79129
# grep 147.50.1.226   /var/log/maillog | grep error
# grep 147.50.1.226   /var/log/maillog | grep Error
# grep 147.50.1.226   /var/log/maillog | grep greeting