The 4.7.1 message is from the server and has nothing to do with Thunderbird. If your ISP is Optus (I say this because I can see loads of comments saying same sort of thing with that ISP), then they have limits. If not Optus, I would say it is the same issue. The message tells you that you have sent alot of mail - what they term as excessive. This sounds like you have either sent more than the allowable quota in a given period, in which case the ban should be lifted after a period of time or your IP address has been flagged as sending spam and the IP address is blacklisted by the receiving server. www.optuszoo.com.au > webmail > quota is anything that is in here including sent mail/trash/etc folders quote: ''What is Optus's email storage policy?'' ''Optus Internet provides 20MB (for Dial-up accounts) or 20MB (for Residential Broadband accounts) or 100MB (for Small Business Broadband accounts) of received email storage (including attached files). A warning message is sent when the amount of stores emails reaches 90% of the limit, and again when it reaches 100%. Once you reach this limit, you will not be able to receive any new email.'' ''You are most likely to exceed this limit if you configure your email program to save a copy of your email messages on the server. It's recommended that you only use this setting if you are temporarily receiving email on a different computer than the one you would normally use - for example, while travelling.'' ''The maximum size of any single email message cannot exceed 10MB. This limit includes all files attached to it. If you attempt to send a messages larger than 10MB, it will be returned to you with an error message '' also see info here: http://forums.whirlpool.net.au/archive/2099053 http://forums.whirlpool.net.au/archive/475366 I would advise that you create an account with eg: gmail. then should you get a similar problem you can use the gmail account to send emails.