Earlier quoted context omitted.
Calendar in the mail client is very useful since invitations are sent by mail. You click accept, the recipient knows you did and the event is added to your calendar. But indeed if you don't use it it does not get in the way, which is perfect.
I read mail first on my phone with K9 (the old UI) a few times per day and accept invitations there to put them on my phone, which is always with me. Then I download mail with Thunderbird a few times per week. I use my domain providers POP3 servers. Yes, they bundle a mailbox with each domain. It's also IMAP and webmail but POP3 is better for my use case.
I'm curious as to why, if you don't mind! Are you specifically looking for "download only once then get rid of it"?