SmtpAppender uses not recomended SmptClient

[DietzeC]

We use the SmtpAppender in a number of projects.

When used with modern mail servers, connection issues are becoming increasingly common.

Until now, the SmtpClient class has been used for sending email, but Microsoft no longer recommends its use:

Important
We don't recommend that you use the SmtpClient class for new development because SmtpClient doesn't support many modern protocols. Use MailKit or other libraries instead. For more information, see SmtpClient shouldn't be used.
https://learn.microsoft.com/en-us/dotnet/api/system.net.mail.smtpclient?view=net-10.0#remarks

Is the project planning to make the recommended switch to MailKit in the near future?

[FreeAndNil]

@DietzeC we are aware of the problem.
The thing is that Mailkit would be a new dependency (or better two). As a base library we don't want new dependencies.
We could:

• move SmtpAppender to a new assembly (similar to Poll: Dropping net462 Support in log4net #267 (reply in thread))
• add an extension point where one could provide a custom mail client
• accept the Mailkit dependency

@fluffynuts @gdziadkiewicz wdyt?

@DietzeC in my company we changed everything from the legacy
SmtpClient to Mailkit. But I can't remember having problems with the old client.
How often do you get which errors with it?

[DietzeC]

Thanks for the reply.

We can't reproduce the specific error in log4net, but we've noticed that emails are being sent from the same application via MailKit with a similar configuration. Problems have been piling up lately. Some of them have occurred after updating to newer versions of Exchange Server. We suspect this is related to SSL/TLS encryption.

We're also considering developing our “own” SmtpAppender with MailKit, but we wanted to ask here first.

Perhaps a forth option would be to create a log4net.mailkit package in addition to the standard log4net package. That way, we wouldn’t introduce a dependency into the main project but could still provide a new MailKit version.

[FreeAndNil]

Thanks for the response.
Until when do you need a "solution"?

[fluffynuts]

my 2c:

If the .net smtp client is both deprecated and doesn't work reliably, drop it completely and introduce a satellite appender package using something better (like mailkit). I'd mark it for deprecation and remove it completely in the next major version.

[DietzeC]

At this point, we still consider this issue to be an inconvenience, but not a problem.

However, to avoid accumulating further technical debt, we would like to see a solution within the next four to six weeks.

[FreeAndNil]

@fluffynuts @DietzeC thanks for your feedback. I will create something like log4net.Ext.Mail after my vacation.

[DietzeC]

@FreeAndNil Thank you in advance, and enjoy your vacation!

[fluffynuts]

@fluffynuts @DietzeC thanks for your feedback. I will create something like log4net.Ext.Mail after my vacation.

Always stepping up @FreeAndNil 👏

[FreeAndNil]

@DietzeC can you test #302?

[FreeAndNil]

@DietzeC you can test with https://github.com/apache/logging-log4net/releases/tag/rc%2F3.4.0-preview.1

• https://www.nuget.org/packages/log4net/3.4.0-preview.1
• https://www.nuget.org/packages/log4net.Ext.Mail/3.4.0-preview.1