If my 2021 IRA contribution is POSTMARKED by April 18, 2022, is that considered within the deadline for 2021?

[mtskainfla]

In other words, let's say it's "in the mail", but not yet posted to my IRA account.

[AnnetteB6]

That is probably a question that you should ask your IRA custodian.  Ultimately it will depend on when they post the contribution to your account and whether the Form 5498 that will be issued to the IRS will show that the contribution was made for the 2021 tax year.  

If they post deposits based on the postmark date, then you should be in good shape.  Otherwise, you might need to make other arrangements with them to get the money deposited sooner.