There is a easy and generic solution.
You are actually calling message-send which calls y-or-n-p to ask you confirm.
So you want to automatically answer "Yes" when being asked the question "Already sent message via mail; resend? (y or n)".
Here is code,
(defvar my-default-yes-no-answer nil
"Usage: (setq my-default-answer '(t . \"question1 pattern\"))"...
"smtp.zenbox.pl" wants the machine from which you're sending mail to have a fully qualified domain name. You can arrange for that either by making sure that (system-name) returns one, or by setting smtpmail-local-domain to an appropriate value.
Much simpler than I realized: just set
mu4e-index-update-error-warning to nil.
Whether to display warnings during the retrieval process.
This depends on the ‘mu4e-get-mail-command’ exit code.
Here's a little function to toggle the setting:
(defun mu4e-toggle-ignore-warnings ()
"Toggle whether or not mu4e reports update process warnings ...