I'm a little confused by your question because Safari and Mail are two separate apps. "Safari Mail" makes no sense to me, but I surmise you're having trouble with the Mail app. Not Safari.
Assuming that is correct:
Press and hold a Shift key while launching Mail from Applications or the Dock. Release the Shift key when Mail opens. That action causes the Mail app to open with no message selected.
If that does not immediately fix the problem, or if it comes back, the fix involves removing that Account from your Mac, followed by adding it again. That action will not affect any messages that remain undeleted on the server.
Instructions are described in Add or remove email accounts in Mail on Mac - Apple Support. You will be following them in reverse order—in other words, remove the email account, and then add that same account again.
If you are unable to even open the Mail app due to its crashing immediately upon launch, you may not be able to access Internet Accounts from within Mail as it describes. Instead, amend those instructions as follows:
Click the (Apple menu) > System Preferences... > Internet Accounts, select the affected account, and then deselect the Mail checkbox for the account. To stop using the account in all apps, make sure the account is selected, then click the Remove button [—].
Please note that merely deselecting the Mail account won't remove it. It just temporarily turns it off in the Mail app, and selecting it again turns it on again in Mail. You need to follow the last (boldface) sentence above to actually remove the account. Adding it again will cause Mail to repopulate its contents. Since messages that remain on the server will be repopulated, you need to find and delete that message first. That can be accomplished through your email service provider's webmail interface, provided it offers one (most do).
Can I go get another mail.app and re install.
No. Mail is incorporated in macOS / OS X.
My iMac is from 2009. I'm running OS X El Capitan, version 10.11.6
That may be relevant.
Be advised that if Mail is crashing due to the way a particular message is formatted, Mail's crashing problem is certain to occur when another similarly formatted message is eventually received. Only Apple would be able to fix that, and the likelihood of them doing so with an update to a macOS version as old as "El Capitan" is just about nil. The only alternative is to upgrade macOS, and if your Mac is too old to be upgraded beyond El Cap then the only alternative would be to buy a newer Mac.
