Fixed in 1.17.3 
Verified on Windows
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I will wait 1.18 release to update my 1.16 Mailspring; i am happy with the 1.16 at the moment.
Hi all, thanks for reporting this and sorry this made it into production. We have automated tests that ensure mailsync can connect to IMAP and SMTP, but this particular problem doesn’t surface until you /log in/ to the SMTP server to send mail. I’ve updated our Github Actions to actually perform the login step so we can verify this never happens again.
This has been fixed in Mailspring 1.17.3 (released Jan 31). The app should have already updated to that version, or you can grab it from the website at https://www.getmailspring.com/download . Thank you for your patience.
If it’s any consolation, this was caused by a major upgrade to a new build pipeline and OpenSSL 3, which puts Mailspring on a solid foundation for another decade of security patches and new features.
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The problem happen again on my 1.17.4-92e9fe28…
I’ll downlod again 1.16 and stay on it for a while I think…
Broken again for me in 1.17.4 @bengotow
Problem solved, it was a SPAM classification of my IP from SPAMHAUS.
Sorry for the bad diagnostic.
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This SMTP authentication issue was introduced in v1.17.2 and fixed in v1.17.3. Please update to the latest version (v1.21.1 as of May 19, 2026) — SMTP should work correctly. If you’re still seeing Error 296 on a current version, please reply with your provider and Mailspring version and we’ll investigate.