Put the jar file into your CATALINA_BASE lib folder. Are you subscribed to 
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From: Zdeněk Henek <[email protected]>
Sent: Friday, March 6, 2026 10:55 AM
To: Tomcat Users List <[email protected]>
Subject: Re: Apache Tomcat 10 Issue




Hi,



> Given that it’s “com/sun/mail” I suspect you will need the original (sun) jvm.



com.sun.mail is not part of jvm.



option 1 - add missing jar file

There is a missing jar. Maybe removed by accident during upgrade?

e.g. 
https://urldefense.com/v3/__https://mvnrepository.com/artifact/javax.mail/mail__;!!F9svGWnIaVPGSwU!oOM6htHDfL1H9B-iSKjSase3t9ZWCgIhs383occ91svXdc09ly0LlscvZIgAIIDZ1VoczFMy_oV8rvmP1VSULF0$<https://urldefense.com/v3/__https:/mvnrepository.com/artifact/javax.mail/mail__;!!F9svGWnIaVPGSwU!oOM6htHDfL1H9B-iSKjSase3t9ZWCgIhs383occ91svXdc09ly0LlscvZIgAIIDZ1VoczFMy_oV8rvmP1VSULF0$>



Look into the old Tomcat 9.0.16 into <tomcat>/lib directory. Maybe the

jar is there and you moved only the war file?





option 2

if you use javax.mail-api it contains only javax. classes

e.g. here 
https://urldefense.com/v3/__https://mvnrepository.com/artifact/javax.mail/javax.mail-api__;!!F9svGWnIaVPGSwU!oOM6htHDfL1H9B-iSKjSase3t9ZWCgIhs383occ91svXdc09ly0LlscvZIgAIIDZ1VoczFMy_oV8rvmPCzrWD9E$<https://urldefense.com/v3/__https:/mvnrepository.com/artifact/javax.mail/javax.mail-api__;!!F9svGWnIaVPGSwU!oOM6htHDfL1H9B-iSKjSase3t9ZWCgIhs383occ91svXdc09ly0LlscvZIgAIIDZ1VoczFMy_oV8rvmPCzrWD9E$>



solution is replace the mail-api with full mail jar file as above.



Get first work the tomcat 9, upgrade to tomcat 10 will need more

changes. The automated migration tool is good but for long term you

definitely need migration. Sometimes it is easy sometimes it is hard,

but Tomcat 9/9.1 will be here a few years so you have enough time.



Regards,

Zdenek Henek



On Fri, Mar 6, 2026 at 4:28 PM Rob Sargent 
<[email protected]<mailto:[email protected]>> wrote:

>

>

>

> > On Mar 6, 2026, at 9:13 AM, Short, William J. 
> > <[email protected]<mailto:[email protected]>> wrote:

> >

> > Sorry for the delay in response.

> >

> > I created a new VM with Server 2025.

> >

> > I didn't exactly recreate current environment because I didn't want to 
> > install versions with known vulnerabilities.  I got it as close as I could. 
> >  I am still having the same problem with sending email notifications.  Here 
> > is my current setup:

> Your rebuild has not included the same version of some jar (or jvm) with 
> which the application was compiled. Hence the NoClassDef error.

>

> Given that it’s “com/sun/mail” I suspect you will need the original (sun) jvm.

>

> But it would be better to recompile the app with new java. And likely a new 
> mail making provider

> >

>

>

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