So are you saying Thunderbird is sending messages without you doing anything using an account which doesn't even exist in your Thunderbird profile? Sounds highly unlikely.

How do you determine Thunderbird is sending unsolicited messages, and not something else?

Is there any possibility I'm infected by a malicious code.

It certainly isn't impossible, but with the limited information available there is no point to speculate.

I Think you didn't understand what I said.

This which I'm saying is that Thunderbird is using my registered account (in mail.att.sch.gr) and is sending messages in another account not registered in Thunderbird that I'm using in my job. This otherone is a gmail account. Obviously this account is in my sending lists. The curious thing is that a huge amount of mailing is forwarded at this other address. I don't know how is happening but it does. Have you an answer? Out of Linux environment it should be a virus. In this case what is it?

I Think you didn't understand what I said.

Indeed.

This which I'm saying is that Thunderbird is using my registered account (in mail.att.sch.gr) and is sending messages in another account not registered in Thunderbird

I don't understand that, and again, I doubt it. You haven't explained why you think it's Thunderbird sending the unsolicited messages. Note, anyone can fake the senders email address when sending an email message.

My dear friend, you can doubt anything you like! This thing is happening and have checked it simultaneously (the two programs functioning, Thunderbird and Chrome). The mails are sent from the address I've described, so if you haven't an answer please don't waste my time with fanfares.

Is the sending account in Thunderbird, a pop or imap mail account?

After starting thunderbird and it says messages sent... 'Menu icon' > 'Activity Manager' Does it say emails have been sent?

Are these 'sent' messages appearing in the 'Sent' folder?

Have you checked your webmail account via a browser to see if there is anything setup to forward emails to the other work account?

Are the emails received into thunderbird mail account and then 'forwarded' to the gmail account?

Do you have any Message Filters? If yes, please check to see if any filter mentions the work gmail account

Observe the content of the received emails in gmail account, do they look like they were written by you? It is possible that someone is abusing your email address.

The mails are sent from the address I've described,

I didn't question that. But you haven't provided any evidence they have been sent via Thunderbird.

if you haven't an answer please don't waste my time with fanfares.

I won't. Good luck.

@Toad-Hall It is a pop account. There is no notification about sent messages, there are no sent messages into the sent folder. My webmail is checked. The problem appears only in Thunderbird in Linux. The forwarded messages are old ones that are received again, as these weren't, and then automatically forwarded to the Gmail account. Unexplainable!!!

re : It is a pop account. There is no notification about sent messages, there are no sent messages into the sent folder.

In addition to the above.... and if you have no 'Message Filters' in that pop mail account auto forwarding emails to your work account. Please check to see if you have any filters that mention your gmail email address.

This means the pop mail account in Thunderbird on that computer is probably not sending anything.

If you have no 'Forwarding' actions created on the webmail account then it is also not the source of the issue.

If you are receiving emails into the gmail account, which claim to be using your personal pop email address, but the content does not look like anything you would send, then it is probably someone abusing your email address.

Have you checked the source code of received email in gmail work account? It should give you some info on IP addresses / servers etc.

RE: when I' m starting Thunderbird, it begins without my action to send messages What happens if you start up Thunderbird in Offline mode?

TEST: You can set to follow or not follow detected online state and choose options here: 'Edit' > 'Preferences' > 'Advanced' > 'Network & Disc space' tab Configure offline settings... click on 'Offline...' button

uncheck 'Automatically follow detected online state' select an option you prefer eg: Offline

Send unsent messages when going online? : select option you prefer, I would suggest 'Ask me' as it gives you the opportunity to check before sending. This will also alert you to the knowledge of something in the 'Outbox' folder.

Download messages for offline use when going offline? If you only download headers, then choose 'yes', otherwise 'no'. Usually people with pop mail accounts will already have downloaded the full email. click on all OK's.

Now when you start up already in offline mode, do you have any emails in 'Outbox' ? Leave Thunderbird operating in offline mode. Yes it means you need to use webmail temporarilly to see new emails, but it would help to see what happens with emails in the gmail webmail account.

As I was afraid of the case of abusing my account I've deleted all the messages in sch.gr account. I've followed your instructions on unchecking 'Automatically follow detected online state', I've checked the option 'Ask me' before sending a message and hoping never happen again. In my gmail account were simply received the messages, nothing else. Didn't thought to see the ip addresses, as I've deleted them because it's a common account for my job. Thanks for your advise. Till now, after deleting the messages in sch.gr, nothing has happened. Despite all these, the problem for me remains! I've believed this could happen only in "violated" Windows... The proof shows that Linux has also problems with vulnerabilities. I hope someday understand what really happened. Thanks again for spending your time.