It occurs to me you said you use this variable in another workflow. If that's the case, then it's a global variable, and therefore the notation for it should be
If that's not the case, then perhaps there's an extra space in the variable contents. If you want to trim it, you can use the Math Expression task right before sending the file to PlanetPress Image:
In the "Math Expression" field:
trim(watch.getvariable("af_ToAddress"))
In the "Store Results in" field:
af_ToAddress
Let us know how it goes.