If you cannot send mail right now, for example because you are currently offline, you can queue the mail, and send it when you have restored your internet connection. You can control this from the The main view.
To allow for queuing, you need to tell smtpmail where you want to store
the queued messages. For example:
(setq smtpmail-queue-mail t ;; start in queuing mode smtpmail-queue-dir "~/Maildir/queue/cur")
For convenience, we put the queue directory somewhere in our normal
maildir. If you want to use queued mail, you should create this directory
before starting mu4e. The mu mkdir command may be useful here,
so for example:
$ mu mkdir ~/Maildir/queue $ touch ~/Maildir/queue/.noindex
The file created by the touch command tells mu to ignore this
directory for indexing, which makes sense since it contains smtpmail
meta-data rather than normal messages; see the mu-mkdir and mu-index
man-pages for details.
Warning: when you switch on queued-mode, your messages won’t reach their destination until you switch it off again; so, be careful not to do this accidentally!