Bill and somdcomputerguy, thank you both for replying.
I want to send emails with the return address <abc@foo.net> not just from FastMail, but from several other email services, too.
Furthermore,I want to send emails with the return address <abc@foo.net> not only to Gmail, but also to other email services that require a DKIM record. Frankly, I am stumped. Therefore, if anyone in this forum could tell me the proper DKIM record I should put into the TXT Record section on my ZoneEdit settings page, I would appreciate it.