[Dovecot] Simultaneous POP login - inbox corrumption

[John Woodell]

When two POP clients connect to the same account,
within a second or two, the inbox can get corrumpted.

Here is a sample of the first few lines
of the corrumpted inboxes...

# 1--
#
#
# From...

#
# >
#
# From..

# 4504
#
# From...

# 7--
#
#
# From...

# 4504
#
# From...

#
# >
#
# From...

# 868--
#
# From...