I have just installed snap version of Mailspring, and there is no option to "Launch on system startup", but, snap creates a symlink on /snap/bin/mailspring, which makes the same as /usr/bin/mailspring, that is, it opens the software. The option to auto start, can simply execute mailspring command , or even mailspring --background one, to auto start with the system.
OBS: This is specifically for snap version, the .deb version starts normally with the system (it has the option to make this under preferences).
June 2019, this has not been resolved. Any workarounds? Having the system execute mailspring -background still opens the GUI client
I think that if you copy the desktop file and place it in ~/.config/autostart, Mailspring will start automatically, but I end up using .deb version because it does not has this bug the theme bug that all snap apps have
Having the system execute bash -c "mailspring -b" at startup works for me with the snap version
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Having the system execute
bash -c "mailspring -b"at startup works for me with the snap version