Apple Mail does not download pop3 after 9 March

Question

From 9 March my Apple Mail refuses to download email from a POP3 connection. I use a MacBook air running 10.10.2. Without any success I have tried the following:
restored from TM backup prior to 9 March - get mail seems to perform correctly but no more emails appear
erase disk and perform full factory install of Yosemite - then setup mail account - all emails up to 9 March reload from server but nothing after
worked with ISP L2 tech for many hours - they are stumped
setup new user on my wife's iMac - this has not had the 9 March security updates applied yet - set up new email accounts - same result with email not being downloaded from 9 March
set up Mulberry and Thunderbird email clients - same result with email not being downloaded from 9 March
tried accessing from work network (MBA is recognised under BYOD settings) that worked perfectly prior to 9/3 but now has same problem - this to prove that ISP connection at home not problem
However emails since 9 March are visible using webmail. Equally emails sent today to this account appear in webmail but not in the Apple client.
I have two pop3 accounts to the same ISP linked to the same account - using the other email address to the same ISP - no problems at all!!
In looking at logs created by Connection Doctor nothing comes up in the All Messages view. But looking at the SocketStremEvents.log the following error messages are shown:
Mar 15 11:46:22 Warwicks-iMac.local Mail[3677] <Debug>: ERROR: Writing failed to socket: 0x6180000b0aa0, error: Error Domain=MCMailErrorDomain Code=1029 "The operation couldn’t be completed. (MCMailErrorDomain error 1029.)"
Mar 15 11:46:22 Warwicks-iMac.local Mail[3677] <Debug>: NSStreamEventHasBytesAvailable -- socket: 0x6080002b3e00 -- thread: 0x61000066abc0
Mar 15 11:46:22 Warwicks-iMac.local Mail[3677] <Debug>: Unscheduled input stream from run loop after waiting to be readable -- socket: 0x6080002b3e00 -- thread: 0x61000066abc0
In looking at Accounts.plist (in ~/Library/Mail/V2/MailData) I note that there are two different settings for the key "UserAllowsInsecureAuthentication" across the two accounts. One is set to false and one to true. my ISP does not use encryption only authentication on POP3 and no authentication on smtp. I have tried setting this to true in both cases but this makes no difference.
Extract 1: this for account exhibiting the inability to receive emails after 9 March:
<string>[email protected]</string>
  </array>
  <key>FullUserName</key>
  <string>Warwick at Sandcastle Consulting</string>
  <key>Hostname</key>
  <string>pop3.paradise.net.nz</string>
  <key>IsFirstTimeSync</key>
  <string>YES</string>
  <key>JunkMailboxName</key>
  <string>Junk</string>
  <key>NotesMailboxName</key>
  <string>Notes</string>
  <key>PortNumber</key>
  <integer>110</integer>
  <key>SMTPIdentifier</key>
  <string>smtp.paradise.net.nz</string>
  <key>SentMessagesMailboxName</key>
  <string>Sent Messages</string>
  <key>ToDosMailboxName</key>
  <string>Apple Mail To Do</string>
  <key>TrashMailboxName</key>
  <string>Deleted Messages</string>
  <key>UserAllowsInsecureAuthentication</key>
  <true/>
Extract 2: this for account that works perfectly:
<string>[email protected]</string>
  <key>ConfigureDynamically</key>
  <true/>
  <key>Hostname</key>
  <string>smtp.paradise.net.nz</string>
  <key>MaxMessageBytes</key>
  <integer>31457280</integer>
  <key>PortNumber</key>
  <integer>25</integer>
  <key>UserAllowsInsecureAuthentication</key>
  <false/>
Needless to say I am stumped. I cannot understand why one email address with a different username but to the same ISP exhibits the problem but not the other.

Answer 1

I have found a fix - delete all the emails using the webmail account. Obviously there was something in one of the emails that was generating an error somewhere - whether on the client or server I am no closer to knowing.
But once all emails were deleted, then test messages started flowing.
Now why didn't I try that a week ago. Ho hum!