IMAP mailbox corruption?

[#1]

Since Xmas, Windows Live Mail (WLM) on several of my machines (both Win 7 and XP; and using different PN IMAP mailboxes) has stalled with “Your IMAP server has not responded within 60 seconds, …” when receiving into their Inboxes. This isn’t network congestion, because once it has happened, it never recovers. Completely removing all traces of WLM (all registry entries and data files in addition to the usual Win Uninstall) and reinstalling makes no difference whatever. Neither does installing a different version of WLM (I have tried V14, V15 and V16).
Some machines get as far as retrieving mail up to a particular date. Others retrieve all headers but only bodies up to a particular date. Still others retrieve all headers but no bodies.
However, each apparently hosed Inbox is readable by PN webmail and by Outlook 2010 and 2013
(To pre-empt replies of “use Thunderbird”, WLM is used on those particular machines because they need to share contact lists and calendars via Microsoft EAS (a.k.a. ActiveSync)

Earlier last year, and nothing to do with WLM, PN told me correctly that I had gone over the 1GB limit and sent me the usage of all mailboxes on my account. I cleared some space, but also noticed that several long removed mailboxes were still shown and as occupying space.
In April my total email storage usage suddenly shot up overnight from its usual 500-700 MB to 1.8GB. PN quite rightly told me to fix it and gave me the usage of the largest mailboxes. The mailbox concerned was showing 1.3GB but Outlook (and PN webmail) said 300MB. We cleared around 80MB. PN then ticketed me to say thanks and that I was now down to a more usual 700MB in total. Hmmmmm.......

These may be two different problems but they sound suspiciously as if they have the same root cause – central mail storage or index (or both) corruption.
Have others (and PN) encountered either or both of these symptoms?

[#2]

Have you tried Compacting the Inbox folders after deleting messages? I think doing this is usually necessary to physically remove deleted messages and free up space.

[#3]

Thanks Spraxyt.
But my second ('monster mailbox') issue appeared to be a monster central IMAP mailbox whose size is only displayed to PN staff; the size of my local cached version was reasonable. Compacting (via WLM's Tools / Options / Advanced / Maintenance compactor tool) is, I guess, only for housekeeping the local cache.
My money is on one or more mails being either:
- corrupted for some unknown reason on PN's central mailstore,
or
- being received into the central store when already in some format which WLM's IMAP protocol stack cannot handle but Outlook's can (and webmail accesses it directly anyway)
Turning on the WLM Troubleshooting log simply shows (amid all the cryptic verbiage) WLM repeatedly trying and failing to get a response from the server when trying to download mail into the mailbox concerned

[#4]

I would recommend going into the email boxes using webmail and try compacting / tidying them up there and then revisit your WLM configuration.