While not strictly mailmerge, http://www.gmayor.com/ManyToOne.htm in one to one mode will emulate a merge to e-mail and allow the selection of an e-mail account to be used for that merge. You could use they filter to determine which of your records to send. Then merge the records for each account separately.

It is not exactly ideal, but short of creating a dedicated custom e-mail process to select the accounts in real time, it's probably as good as you are going to get.

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Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com