Mail App downloads Emails after Installing

Hi, i have a problem with the Mail app. I just want to use it to send scheduled Emails. I am using Nextcloud on a raspberry. I have mail accounts with countless emails in it. I am missing an option to only manually receive email from my account and/or a limited number of emails and/or the possibility to switch off receiving at all. Am i missing an option or shpuld i open an issue on github?

Another problem were when i tried to install the Mail app with receiving all my Emails which took very long, the Mail App arbitrarily changed headers of countless Emails to “important”. It was a lot of work to change that back afterwards. The default settings should forbid that. And there should be a bigger warning that that functionality changes mail headers.