help with t/s retrieval 'hangs' disconnect for inactivity

[voytek at sbt.net.au]

I have Dovecot 2.1.17 on Centos, all working well

since yesterday one user complaining he can not retrieve emails, TBird
mail client says like '1 of 9' and, doesn't go any further.

that's on several of his PCs using TBird, on a LAN behind f/wall, quite
physically remote to server, server in Sydney, remote client is SEAsia

looking at logs I see emails access, looking at server Maildir , new
emails get moved from 'new' to 'cur'

what else can I do from server end ?
what else should be done from user end ?

Jun 19 11:28:31 pop3(it at aaaa.com.au): Info: Disconnected for inactivity in
reading our output top=0/0, retr=1/0, del=0/56, size=6562805
Jun 19 11:29:20 pop3(it at aaa.com.au): Info: Connection closed top=0/0,
retr=1/0, del=0/56, size=6562805