I have been using Thunderbird as my default email for years and have run into my share of issues. Out of nowhere, starting this morning, I am unable to send emails from Thunderbird at all. I spent about 2 hours on the phone with my service provider trying to figure it out. The unfortunate conclusion I have come to, is that unless you can fix this issue for me, I will be using Outlook going forward. Let me be clear, I HATE OUTLOOK! but I am running out of options other than to throw away the email address I have been using since about 1990. This is the message I get when the email does not send. This particular one was to trs.ky.gov, but it doesn't matter who the recipient is, the only ones that don't get this message are those I send to myself. Your message contains invalid characters (bare line feed characters) which the email servers at trs.ky.gov don't support. reb Office 365 julie.soard Action Required Recipient Bare line feeds unsupported How to Fix It It appears that your email program added invalid characters (bare line feed characters) into your message when you sent it. Try using a different email program to send your message. If the problem continues send this non-delivery report (NDR) message to your email admin for assistance. Was this helpful? Send feedback to Microsoft. More Info for Email Admins Status code: 550 5.6.11 This error occurs when the email program or device used to create or send an email message adds bare line feed characters into the message. When bare line feed characters are included in a message, the SMTP protocol chunking feature is required to transmit the message between email servers. Chunking uses the SMTP protocol BDAT command, but the recipient's email server doesn't support the BDAT command. To fix the issue, the sender should send the message using an email program or device that doesn't add bare line feed characters to messages. Most modern email programs don't add bare line feed characters. If the sender is using a device like a fax machine, printer, or scanner, it's likely an older model that doesn't support the SMTP protocol BDAT command. To fix this error, you'll need to replace the device with a newer model that supports the BDAT command. The industry standard recommendation to support SMTP chunking was published in 1998 so most devices manufactured during the last decade support this feature. Another option is for the email admin at the recipient's domain to upgrade their email servers to servers that support the SMTP protocol BDAT command. Most modern email servers support BDAT; however, some free and older email servers don't support it. For more information see Fix email delivery issues for error code 5.6.11 in Office 365. Original Message Details Created Date: 8/2/2022 9:35:48 PM Sender Address: reb@isoc.net Recipient Address: julie.soard@trs.ky.gov Subject: Fwd: documents for retirement Error Details Reported error: 550 5.6.11 SMTPSEND.BareLinefeedsAreIllegal; message contains bare linefeeds, which cannot be sent via DATA and receiving system does not support BDAT DSN generated by: SN6PR08MB5327.namprd08.prod.outlook.com