mail-bury-selects-summary

mail-bury-selects-summary is a variable defined in `sendmail.el'.
Its value is

t

Documentation:
If non-nil, try to show Rmail summary buffer after returning from mail.
The functions M-x mail-send-on-exit or M-x mail-dont-send select
the Rmail summary buffer before returning, if it exists and this variable
is non-nil.

You can customize this variable.