Sourced from MailKit's changelog.

MailKit 3.1.0 (2022-01-14)

• Fixed NTLM to always prefer the supplied domain over the TargetName or TargetInfo.DomainName. (issue #582)
• Updated GMail and Outlook.com SSL certificate info.
• Added a new SslCipherSuite property to each client that allows developers to get information about the SSL/TLS cipher suite that was negotiated with the server. (pull #1312)
• Reduced string allocations in SmtpClient's EHLO capability parsing logic.
• Default ProtocolLogger.RedactSecrets to true for added added security.
• Added work-around for parsing malformed GMail ENVELOPE responses that reverse the name and address components of the Sender address. (pull #1319)
• Added net6.0 to the list of TargetFrameworks.

MailKit 3.0.0 (2021-12-11)

• Removed APIs marked as [Obsolete] in 2.x.
• Simplify Fetch()/FetchAsync() APIs by using a new IFetchRequest parameter instead. Made previous APIs into extension methods to aid in porting from 2.x.
• Replaced Add/Remove/SetFlags() APIs with Store()/StoreAsync() and simplified the APIs by using a new IStoreFlagsRequest parameter. Made previous APIs into extension methods to aid in porting from 2.x.
• Replaced Add/Remove/SetLabels() APIs with Store()/StoreAsync() and simplified the APIs by using a new IStoreLabelsRequest parameter. Made previous APIs into extension methods to aid in porting from 2.x.
• Simplify Append()/AppendAsync() APIs by using a new IAppendRequest parameter instead. Made previous APIs into extension methods to aid in porting from 2.x.
• Simplify Replace()/ReplaceAsync() APIs by using a new IReplaceRequest parameterinstead. Made previous APIs into extension methods to aid in porting from 2.x.
• Updated SmtpClient.Send()/SendAsync() methods to return a string. (issue #1161)
• Added support for the SCRAM-SHA*-PLUS SASL mechanisms. (issue #950)
• Added authzid support for SCRAM SASL mechanisms.
• Added support for the ANONYMOUS SASL mechanism.
• Added support for an HttpsProxyClient. (issue #1251)
• Added AcceptedKeywords and PermanentKeywords to IMailFolder. (issue #1256)
• Rewrote NTLM support based on official specs. Now supports channel-binding and using the default system credentials.
• Modified ImapFolder.Fetch(int, int, ...) to shortcut if ImapFolder.Count == 0.
• Updated SmtpClient to append an ORCPT arg to RCPT TO commands and to hex-encode the ENVID parameter value.
• Improved/simplified logic for ranking SASL authentication mechisms for each client.
• Added SaslMechanism.ChallengeAsync() to facilitate future SASL mechanisms that may need to make network requests such as Kerberos/GSSAPI and perhaps even future/custom OAuth2 implementations.
• Always set SearchResults.Count/Min/Max properties if we can.
• Throw TimeoutException is case of a network time out. (issue #1269)
• Fixed parsing of IMAP flag lists to handle lowercase flag names. (issue #1277)
• Use OrdinalIgnoreCase when comparing "EARLIER" atom token.
• Avoid unnecessary string copies. (issue #1292)
• Drop support for .NET 4.5 and replace it with .NET 4.5.2

... (truncated)

• 8aec015 Bumped version to 3.1.0
• 4aef76a Added more outlook.com certificates
• 0287ab2 Updated GMail and outlook.com SSL certificate info
• 1f99b98 Fixed ProtocolLogger unit tests for RedactSecrets change
• bfbe2a2 Updated copyright notices
• 39c57c1 Update main.yml
• 301cfc7 Build MailKit and MailKitLite for net6.0 as well
• 42e6441 Bumped MimeKit dependency
• e3a4549 Merge branch 'master' of https://github.com/jstedfast/MailKit
• 5459625 Added compat for parsing a malformed sender address that comes in the fetch c...
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