There are two safe modes; Thunderbird's own, and your operating system's. When using the operating system safe mode, you need to choose "Safe mode with networking". HOW DO I DO THAT? I AM NOT AN IT

Thunderbird's own safe mode disables add-ons and UI tweaks. It should not interfere with connectivity and we generally suggest it to eliminate possible sources of trouble. THIS IS CHINESE FOR ME . I DON'T UNDERSTAND.

http://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10 https://support.mozilla.org/en-US/kb/safe-mode-thunderbird

But over and above that, who is your email provider? And your ISP? THE EMAIL PROVIDER IS ONE.COM - I DO NOT KNOW WAHT ISP MEANS

You have started five separate threads now. That makes it very hard to try to help you.

There is no chat, no telephone help line.

ISP = internet service provider. The people you pay for your internet connection. The internet is quite useful for finding answers like this.

I gave links on how to get to safe mode in both Thunderbird and Windows.

Your outgoing settings use 2525 but one.com's website says you should be using 465. I would try changing one of them to 465 to see if that is the problem.

I can work with Gmail.com but since last saturday I cannot send mails through Thunderbird in combination with one.com anymore. Receiving works fine.

is there an easier way to communicate with your helpdesk? Now you send me questions through mail that I have to answer on the helpdesk. Not the easiest way I would say.

This page https://www.one.com/en/support/guide/mail/setting-up-windows-mail suggests 465/SSL for smtp. Do you know where the unusual 2525 setting came from? NO I DON'T KNOW WHERE IT COMES FROM - WHY IS IT UNUSUAL?

I DID NOT CHANGE ANY SETTINGS LAST SATURDAY. ALL OF A SUDDEN BOTH MY COMPUTER USING THUNDERBIRD DO NOT WORK ANYMORE FOR OUTGOING MAIL. MY IPHONE USES ONE.COM AS WELL AND STILL WORKS NORMALLY.

One.com may have changed something. Do you use any software that scans your outgoing messages?

It would be useful to see what settings are being used on the phone.

The 2525 is very unusual and would be my first concern. Nothing on the one.com website tells us why you would have this setting.

Email needs standards so it can work anywhere, any time, on any device. One of those standards is to use specific port numbers for communicating. 25, 465, 587 are typical standard port numbers for sending email. 2525 is not a recognised port for regular email, but it does look like the sort of port number a mail scanning program might use.

Or maybe one.com stopped using it but failed to explain to you what you need to do.

I understand that you have no solution. I contacted the helpdesk of one.com as well. We'll see what they offer. Should I change Thunderbird for another mailprogram in order to make things work again?

You need the right settings. Tell one.com what you are using.

I changed the setting for one of the mailboxes to 465 but that doesn't change anything. The program doesn't seem to be able to connect.

Duplicate threads https://support.mozilla.org/en-US/questions/1147602 https://support.mozilla.org/en-US/questions/1147595 https://support.mozilla.org/en-US/questions/1147599 https://support.mozilla.org/en-US/questions/1147395

are being closed in favor of the canonical thread 'https://support.mozilla.org/en-US/questions/1147517.