Also tried bypassing the wifi and connected by ethernet. Same issue.

When you say cannot be sent or erceived is there any error message ? If yes, it would be helpful if you uploaded an image of what it says.

In the meantime check both of the following.

Firewall: Access Firewall and make sure Thunderbird program is set up as allowed, so nothing gets blocked by firewall. Do not just switch off Firewall, you should have Thunderbird set up as allowed.

VPN Check computer for a VPN which has been enabled. Many Anti-Virus products now offer a VPN and in some cases I've discovered it was auto enabled without user knowledge. Many servers do not like VPN as it hides the IP address. Servers then think you might be a hacker and so may block access. Switch off VPN or set up split tunnelling and get thunderbird to use it.

Hi, Checked my Anti-Virus settings, and Thunderbird is allowed. Checked Widows Firewall and it is disabled by the Anti-Virus program. Thunderbird would not connect. Turned off the Anti-Virus, and checked that Windows Firewall is allowing Thunderbird, still no connection. My Anti-Virus does not have a VPN. Disabled my VPN, and still no connection.

My Optus Webmail works, so Optus says it's a Thunderbird problem. Got them to escalate the issue, but they still said there was no problem on their end.

Attached are the pop up I get when trying to download new emails (Firefox Optus Error-4.jpg) and an error message, which, I admit I don't understand (Firefox Optus Error-3.jpg)

Sadly, I am dumping Thunderbird. After two months of trying to get it to work, I give up. My last resort was this post, but as I have gotten no help, I feel that I have no option to start afresh. New email client, new email provider. Am now in the process of transferring everything over. Will delete Thunderbird when done. Thunderbird was once geat - no more.

I would certainly support the new email provider. I have been seeing folk have from Optus for months now and Optus appear to be very good at just "sending the fool further" as my Grandmother used to tell us.

But I will have a stab here. I do not claim any special knowledge, nut I do harbor quite a lot of feeling against Optus so I feel I need to mention that bias early on here. Don't just disable or check the antivirus treat it like it is the offending cause of your issues until proven otherwise. For those that have the option of a wired connection to the internet the following is probably the preferred approach.

• Restart Thunderbird with add-ons disabled (Thunderbird Troubleshooting Mode). On the Help menu, click on "Troubleshoot mode". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

However your error messages do offer a vague clue. The error is about exceeded connections not an inablitlity to connect, those error are generally only apparent with IMAP mail accounts which routinely use multiple connections to multiple folders. (Thunderbird has a limiting option in account settinfs > server Settings > adbanved for that) Unfortunately these multiples connections often have an upper limit around 5 and it is total connections that have that limit. Historically some providers have lower limits than 5 (Yahoo I think is three) and those that own products that have Apple logos tend to have more issues than most the an iphone will simply connect over and over until the server refuses further connections, so while the apple product is snappy and fast. Other devices do not stand a chance.

The other connection limit exceeded is one I have seen when some servers are not authenticated or you have attempted to authenticate using another internet providers service. Instead of telling you you need to log in they tell you you have exceeded your allowed connections (It is Zero but they do not say that.)

So let's try your system in safe mode, while you have it in that mode try Thunderbird in troubleshooting mode as well just to see if that causes an improvement.

There is a problem with Optus, just based on the numbers of their customers that end up in these forums with vague issues around connecting and their exceedingly small representation in Thunderbird users. I am not sure where the issue it lies, it may even be in their sale of internet security software as part of their packages.

McAfee which forms a core of their offerings to consumers has been blocking emails since at least 2012. There are some documented issues here https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#McAfee

Basically no one else in the world has used Port 25 to submit mail from consumers for decades because Port 25 was the source of most of the SPAM in the 1990s I started writing that and went to check the settings on the Optus site. Lo they have changed. So do you settings match the old ones from the 1990s or the new ones Optus have posted.https://www.optus.com.au/support/answer/using_an_email_application_with_optusnet_email_1434

While doing that bit of checking I observed that Optus email is now provided by the ATMAL group. The same people that took over iiNet email. There is now an anual chage for iiNet email addresses in addition to the subscription. When I inquired about the server name mail.optusnet.com.au this what when was returned

Tracing route to imap-waratah.au-east.atmailcloud.com [54.253.41.174]

However the IP address 54.253.41.174 is registered to Amazon AWS so atmail is renting capacity on Amazon.

This page suggests the transition to atmail has been occurring for around a year. https://www.optus.com.au/support/answer/we_re_making_improvements_to_our_optusnet_service_20318