Creare un nuovo script con Script Editor:

(*
Speed up Mail.app by vacuuming the Envelope Index
Code from: http://www.hawkwings.net/2007/03/03/scripts-to-automate-the-mailapp-envelope-speed-trick/
Originally by "pmbuko" with modifications by Romulo
Updated by Brett Terpstra 2012
Updated by Mathias Törnblom 2015 to support V3 in El Capitan and still keep backwards compability
Updated by @lbutlr for V5 and Container folder in High Sierra and use du
https://gist.github.com/ttscoff/3728277
*)

tell application "Mail" to quit
set os_version to do shell script "sw_vers -productVersion"
set mail_version to "~/Library/Mail/V2/Maildata/Envelope\\ Index"
considering numeric strings
if "10.10"  os_version then set mail_version to "~/Library/Mail/V3/Maildata/Envelope\\ Index"
if "10.12" < os_version then set mail_version to "~/Library/Mail/V4/Maildata/Envelope\\ Index"
if "10.13"  os_version then set mail_version to "~/Library/Containers/com.apple.mail/Data/Library/Mail/V5/Maildata/Envelope\\ Index"
if "10.14"  os_version then set mail_version to "~/Library/Containers/com.apple.mail/Data/Library/Mail/V6/MailData/Envelope\\ Index"
if "10.15"  os_version then set mail_version to "~/Library/Containers/com.apple.mail/Data/Library/Mail/V7/MailData/Envelope\\ Index"
end considering

set sizeBefore to do shell script "du -h " & mail_version & "| awk {'print $1'}"
do shell script "/usr/bin/sqlite3 " & mail_version & " vacuum"
set sizeAfter to do shell script "du -h " & mail_version & "| awk {'print $1'}"
display dialog ("Mail index before: " & sizeBefore & return & "Mail index after: " & sizeAfter & return & return & "Enjoy the new speed!")
tell application "Mail" to activate

Su macOS 10.14 Mojave (e successivi), prima di eseguire lo script dall’app, aggiungere Script Editor all’elenco presente in Preferenze di Sistema > Sicurezza e Privacy > Privacy > Accesso completo al disco; conclusa l’operazione è possibile rimuovere l’app dall’elenco.


fonte: 1, 2