How can I introduce a delay in the filter?

I'm using this filter to save emails I get using getmail to a local 
maildir, and to forward them to an external gmail address using msmtp

My problem is that from time to time the server fails in forwarding them 
with the following message:

550-Requested action not taken: mailbox unavailable
550 Failure sending mail. Try again later

I'm not sure why is this happening, but I usually see this when several 
emails go out in a short period.
I suspect either the SMTP server or the recipient gmail account might be 
temporarily blacklisting me.

So, I was thinking in introducing a short delay after or before sending 
the email to msmtp, though I'm not sure how to do it.

I have now: to "| /usr/bin/msmtp [email protected]"


Could you suggest how to introduce a small delay? 1 second would be OK

Can I do for example: to "| sleep 1 & /usr/bin/msmtp [email protected]"

or should I do a loop to waste time? how could I control the timing of 
that loop?


Thanks in advance


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