How much memory do my `smtpd` processes actually use?

You cannot use the "top" output -- the RSS value measures how of that virtual address space is in RAM being actively used and is not paged out or a candidate for being paged out.
These pages may still be shared. The typical per-process marginal cost for smtpd(8) is barely over 8kB when not using Berkeley DB tables.
Just add the data segment (initialized portion may be copy-on-write), heap and stack sizes.
In my case I am using BerkeleyDB, so pmap pid_of_an_smtpd_process shows:

08048000    308K r-x--  /usr/libexec/postfix/smtpd
08095000     12K rw---  /usr/libexec/postfix/smtpd
bf989000     88K rw---    [ stack ]

© by Ralf Hildebrandt
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This file was last modified 30. Aug 2007 by root

How much memory do my smtpd processes actually use?

How much memory do my `smtpd` processes actually use?