ISP server is always full of email messages because my Macmail v.11.2 does not delete them from the server when I download them. ISP says I can change this. I can't find a place to select this option.
iMac, macOS High Sierra (10.13.3)
That is the way IMAP works. IMAP keeps your email on the server so that it can sync your emails across multiple devices. If you change to POP then your email will be downloaded to the first device to access the email account. POP is an older protocol often not used in favor of IMAP because people now access their email from multiple devices.
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Downloading email messages does not delete them from the ISP server
