This problem seems to afflict a few people, but I can't find an answer that completely matches my predicament.
I am running Mail 9.3 on a 2008 Macbook Pro (time for a new one!), OS 10.11.6.
I still use an old ntlworld email address. I have detected that my ntlworld account is continually downloading emails - but I'm not sure where to: my total number of emails doesn't seem to increase When I switch the account off, my available hard drive space stays the same. When I switch it back on, it starts decreasing - I now have only 3GB free. A few weeks ago it was something like 26GB free and I haven't done anything to use up the space that I know of.
I am using ntlworld mail servers with no TLS certificate.
The account type is IMAP.
The trash and junk messages are set to be deleted from the server after a month.
The IMAP Path Prefix is set to INBOX.
The Port is 993 using SSL.
Password authentication is on.
Using the IDLE command is switched on.
The mailboxes on my iPhone and iPad work perfectly.
I am surprised Mail shows the mailbox as the IMAP prefix path should be blank; a restart of Mail will be need if you change the prefix.
In Mail locate a folder listed under the ntlworld.com account, press Ctrl + mouse button, select Get Account Info and under Quota Limits note the in use value. Does this value account for the issue with storage? Sign into webmail and confirm the noted in use is an approximate match to that shown under Quota (under folder view toward bottom left).
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Maybe Mail's Connection Doctor can shine a light on the cause. In Mail select Window > Connection Doctor, then Log Connection Activity and Show Detail. Hopefully the logs will show exactly what is happening to consuming the disks space. Should that not work then consider disabling download of attachments to see if that alleviates the issue by:
select Mail > Preferences…
select Accounts and then the problem account
select Down Attachments under Account Information and set it to None
restart Mail and see if that resolve the issue NB the above change should be consider temporary until attachment causing issue is removed
With attachments disabled, the ntlworld account now no longer seems to be continually downloading (possibly because I did not quit mail after disabling - once I quit the app and restarted, the downloads stopped).
In Activity Monitor "ReportCrash" keeps appearing and disappearing.