What would cause your email program to suddenly not send emails? I have Windows Mail and all of a sudden I cannot send emails although emails are received as usual. I have not changed anything at all on my computer. Just turned it on this morning as usual and when I tried to email my wife it wouldn't send and after a half minute or so I get an error message with some mumbo jumbo about SMTP protocols which again, I haven't changed. I tried sending test emails to other email addresses as well in case there was something going on with her email but they wouldn't send either.

depending on who your email provider is, they may have changed something on their end.

for instance I have att.net email address, but it turns out it is actually yahoo mail.
They added a layer for security and I can no longer use my att.net log in to get mail from my outlook program. I have to go in and generate a new password just for my email program to log into the smtp server to get my mail.
took me forever to figure it out.

you will have to verify at your providers home page and see what they say about email setup and make sure your program does what they need. see if you can see it via a browser instead of a mail program.
they could have also changed ports or it could just be down today.
lots of possiblities.

I had the same issue a few weeks ago. My emails were being flagged as "spam". The problem - whatever it was - was resolved within a few days. I have no idea what the issue was. But I can tell you that I was sending long emails with "links" which may have been the issue BUT when I sent the same to my business email it went through with no problem. The error message did say it was an issue with the recipient's server but it seemed off that a few different folks would all be on the same server. Wait a couple days - it'll probably be resolved.

What version of Windows? Windows 10 or Windows 7? Try sending email again and take a screenshot of the "mumbo jumbo" SMTP message. Could be a lot of things. Throw us a bone and provide more info.

Big difference between emails being flagged as spam and not being able to send at all, which it sounds like the issue the OP is having.

You need to access your email settings within the app. You need to know your email providers requirements. smtp needs to know the address, for instance smtp.google.com

You need to know the port number, for instance 25, 465

You need to know whether encryption is required, and if it is, is it tsl or ssl and set it correctly.

Check those settings, google your email providers (your domain name) requirements, and change them as required.

If you've done all this and still have problems, remove and reinstall your email client, sometimes things just get corrupted. Or try another email client, Thunderbird is the old fall back favorite.

This video will sort of familiarize you with the mail app and settings.

Solved. It turns out the outbound server port number was changed. I didn't change it so if my email service provider changed it for some reason they didn't send any message that such a change was being made. Thanks to all for the replies.

Gotta love windows, I've been listing some stuff lately and a buyer said I sent you a response on C/L but you never responded sp he responded to a FB MP posting on the same item.
I later checked and sure enough the response's weren't coming through but they had been 4 weeks earlier. A little googling led me to discover that SOME incoming mail was being directed to an inactive outlook program.

I sure didn't change anything but iI suspect the last update did