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SMTP authentication failures in version later than 21.4

I’m trying to validate a microsoft365 email in a smtpclient, server “https://smtp.office365.com/”, port 587. In version 21.4 authentication works, when going up to version 21.5 it does not authenticate anymore.

Any idea what might be going on? Should any parameter be changed after version 21.4?

Hello @Berkshire,

can you please provide us a code example you use? Thank you.


Aspose.Email Version 21.7:
SmtpClient(“smtp.office365.com”, 587) { UserName: email, Password: passw }; - DO NOT WORK
SmtpClient(“smtp.office365.com”, 587, SecurityOptions.SSLExplicit ) { UserName: email, Password: passw }; - WORKS

Version 21.4
SmtpClient(“smtp.office365.com”, 587) { UserName: email, Password: passw }; - WORKS

I noticed that in version 21.7 putting SSL Explicit worked for office365, but why is this necessary?
Some servers happen the other way around, they work on port 587 without SSLExplicit…

Any ideia? Thanks.


SSLExplicit - Uses the STARTTLS command to start SSL/TLS connection.
SSLImplicit - Establishes a SSL/TLS connection first.

STARTTLS is command that begins the conversation in plaintext, and if possible, upgrades to TLS. It is the preferred method as one port can handle both plaintext and TLS.

STARTTLS was standardized on port 587. Most of email clients use port 465 for Implicit SSL/TLS and 587 for Explicit SSL/TLS.