FossaMail v27.0.0/38.0.0 Can't Receive or Send Messages Topic is solved

[DraftStan]

I'm currently running version 25.2.4 of FossaMail, on Windows 10 Pro, version 1607, build 14393.693.
After updating to version 27.0.0/38.0.0, FossaMail no longer sends or receives messages. It fails to connect to the incoming mail server, and loses connection to the outgoing server on every attempt to send a message. Server settings remain the same after the update, but even trying different authentication methods results in the same inability to send/receive messages.
I have the following add-ons installed and enabled.
ImportExportTools 3.2.5
Lightning 25.2.4
Restart Thunderbird 1.0
Send Later 6.1
I have attempted to update to version 27.0.0/38.0.0 with the above add-ons installed and enabled; with the add-ons disabled; and with the add-ons removed. The same inability to send/receive messages results in all three cases. FossaMail's ability to send/receive messages is restored in every case by reverting to version 25.2.4.
I'd appreciate any help or ideas anyone can offer to resolve this problem.
Thanks!

[BlueSnow]

Maybe security software, such as av or a firewall or such, that the new version may need to be whitelisted on?

If you're trying the 64 bit version, maybe try 32?

Any particular error messages?

[DraftStan]

FossaMail is allowed by our AV software (Symantec Endpoint Protection) and is not being blocked by a firewall.
I tried the 32-bit version as per your suggestion and I get the same result of not being able to send/receive messages.
The only error message I receive is the following upon attempting to send a message.
Send Message Error
"Sending of the message failed.
"The message could not be sent because the connection to Outgoing server (SMTP) mailserver was lost in the middle of the transaction. Try again."
I receive this message on every attempt to send an e-mail. Server settings for my e-mail account remain the same between the currently-installed version (25.2.4) and the new version (27.0.0/38.0.0).

[BlueSnow]

Sorry, I'm out of ideas, hopefully an expert will speak up.

I'd be curious whether you only have the one account, or if you have multiple accounts and it happens on all of them. Either way, I still wouldn't have a clue to be honest.

[Moonchild]

It does sound like something is interfering with your outbound connection, and I do suspect Symantec's interference. The message you get is that the connection is initiated and some connection is established, but the dialogue to send e-mail never finishes and the connection is closed (either by the server or by something in the middle) before it completes. Can you please double-check for any interception of connection and the endpoint configuration? FossaMail 27/38 is based on different front-end code and may be treated differently; depending on what mail server you are trying to send to, it may also need other port access for e.g. OAuth2.

[DraftStan]

BlueSnow, I only have one account, and it's my work e-mail account.
Moonchild, I'll work with our network admin to follow up on your suggestions. He's been otherwise engaged and hasn't really had the extra time to properly sort out what might be happening.

Thanks to you both for your help!

[DraftStan]

After speaking with our network admin and learning that we're using Exchange 2007 for our e-mail, I followed the advice in this post viewtopic.php?f=34&t=14613, and enabled the deprecated ciphersuites in the config editor. That did it! I am now able to send/receive messages without difficulty.
Thanks again for the help!