I use the Outlook desktop app to check my email for an Outlook.com account. Of late, whenever I send an email using Outlook desktop, I get the error '501 5.1.3 Invalid address' . If I log into Outlook.com through a browser and send the same email to the same email address, it gets sent without issue. This doesn't just happen with one email address, but with multiple addresses.

Has anyone seen this before?

For reference, I have this setup as a POP account.

Incoming Server: pop3.live.com

Outgoing Server: smtp.office365.com

Harassment is any behavior intended to disturb or upset a person or group of people. Threats include any threat of suicide, violence, or harm to another. Any content of an adult theme or inappropriate to a community web site. Any image, link, or discussion of nudity. Any behavior that is insulting, rude, vulgar, desecrating, or showing disrespect. Any behavior that appears to violate End user license agreements, including providing product keys or links to pirated software. Unsolicited bulk mail or bulk advertising. Any link to or advocacy of virus, spyware, malware, or phishing sites. Any other inappropriate content or behavior as defined by the Terms of Use or Code of Conduct. Any image, link, or discussion related to child pornography, child nudity, or other child abuse or exploitation. Hello JenrG
I'm Diane, an Office Apps & Services MVP specializing in Outlook, and I’m happy to help you today.
Only a few addresses are a problem?
If you delete the addresses from the autocomplete list when they come as you type them in, then choose them from contacts (or type them in), do the messages send?
Incoming should be outlook.office365.com but that would not affect sending mail. (Server settings are in Outlook.com's Settings (gear icon) > View all Outlook settings > Mail > Sync email).
--
Diane Poremsky

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Harassment is any behavior intended to disturb or upset a person or group of people. Threats include any threat of suicide, violence, or harm to another. Any content of an adult theme or inappropriate to a community web site. Any image, link, or discussion of nudity. Any behavior that is insulting, rude, vulgar, desecrating, or showing disrespect. Any behavior that appears to violate End user license agreements, including providing product keys or links to pirated software. Unsolicited bulk mail or bulk advertising. Any link to or advocacy of virus, spyware, malware, or phishing sites. Any other inappropriate content or behavior as defined by the Terms of Use or Code of Conduct. Any image, link, or discussion related to child pornography, child nudity, or other child abuse or exploitation. Harassment is any behavior intended to disturb or upset a person or group of people. Threats include any threat of suicide, violence, or harm to another. Any content of an adult theme or inappropriate to a community web site. Any image, link, or discussion of nudity. Any behavior that is insulting, rude, vulgar, desecrating, or showing disrespect. Any behavior that appears to violate End user license agreements, including providing product keys or links to pirated software. Unsolicited bulk mail or bulk advertising. Any link to or advocacy of virus, spyware, malware, or phishing sites. Any other inappropriate content or behavior as defined by the Terms of Use or Code of Conduct. Any image, link, or discussion related to child pornography, child nudity, or other child abuse or exploitation. There seems to be a bug in autocomplete - the addresses sometimes get corrupted.
Deleting the address from autocomplete then selecting it from Contacts always works - typing it in has always worked for me, but a few users said they need to create a contact for the address, just typing it in after deleting it from autocomplete did not work. --
Diane Poremsky

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Harassment is any behavior intended to disturb or upset a person or group of people. Threats include any threat of suicide, violence, or harm to another. Any content of an adult theme or inappropriate to a community web site. Any image, link, or discussion of nudity. Any behavior that is insulting, rude, vulgar, desecrating, or showing disrespect. Any behavior that appears to violate End user license agreements, including providing product keys or links to pirated software. Unsolicited bulk mail or bulk advertising. Any link to or advocacy of virus, spyware, malware, or phishing sites. Any other inappropriate content or behavior as defined by the Terms of Use or Code of Conduct. Any image, link, or discussion related to child pornography, child nudity, or other child abuse or exploitation.

I tried it, but it didn't quite work.

1. Deleted the autocomplete for one email address.

2. Started a new email.

3. On the email, clicked the "To" button. Selected the name from the contact list. Hit OK. Sent the email.

4. Immediately received an email from "System Administrator"

Your message did not reach some or all of the intended recipients.

Subject:     XXXXXXXXXX

Sent:  7/6/22 11:13 AM

The following recipient(s) cannot be reached:

XXXXXX XXXXX on 7/6/22 11:13 AM

Server error: '501 5.1.3 Invalid address'

5. Hit Fwd on the original email.

6. Copied the email address from the original email and pasted it into the "To" box.

7. Resent the email.

8. Seems good. No message from "System Administrator"

One thing I wanted to mention, is that every time I get this System Administrator message, it only shows up in the desktop app. If I login to the browser app, the message is not there.

Thanks again for your help.

Harassment is any behavior intended to disturb or upset a person or group of people. Threats include any threat of suicide, violence, or harm to another. Any content of an adult theme or inappropriate to a community web site. Any image, link, or discussion of nudity. Any behavior that is insulting, rude, vulgar, desecrating, or showing disrespect. Any behavior that appears to violate End user license agreements, including providing product keys or links to pirated software. Unsolicited bulk mail or bulk advertising. Any link to or advocacy of virus, spyware, malware, or phishing sites. Any other inappropriate content or behavior as defined by the Terms of Use or Code of Conduct. Any image, link, or discussion related to child pornography, child nudity, or other child abuse or exploitation.


One thing I wanted to mention, is that every time I get this System Administrator message, it only shows up in the desktop app. If I login to the browser app, the message is not there.
What are you using for antivirus?
Is the address in the bounce message properly formatted? No extra leading or ending spaces or odd characters? --
Diane Poremsky

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Harassment is any behavior intended to disturb or upset a person or group of people. Threats include any threat of suicide, violence, or harm to another. Any content of an adult theme or inappropriate to a community web site. Any image, link, or discussion of nudity. Any behavior that is insulting, rude, vulgar, desecrating, or showing disrespect. Any behavior that appears to violate End user license agreements, including providing product keys or links to pirated software. Unsolicited bulk mail or bulk advertising. Any link to or advocacy of virus, spyware, malware, or phishing sites. Any other inappropriate content or behavior as defined by the Terms of Use or Code of Conduct. Any image, link, or discussion related to child pornography, child nudity, or other child abuse or exploitation.