The default BLAST setup is a very conservative approach: it needs
more than 1000 messages to start working.
I restarted my own BLAST database multiple times, sometimes I
had very good results after 200 messages, sometimes it needed
about 300 to 400 messages. In all cases this is not a 'all around
careless' situation, it needed some "this is not spam"
clicks. After about 600 messages it was almost errorfree for me
(about 500 mails per week).
Due to the nature of BLAST these values can not be generalized
to every user. (And that is why I choose the conservative default
value). But they are a reference if you want BLAST to start earlier.
It is possible to limit the total number of messages in BLAST
to less than 4000 (the default). This will speed up the load time/scanning
start delay, reduce the time needed to batch the BLAST files and
save some disk space.