In case anyone is interested:

Say I want to allow multiple recipient delimiters, possibly more than one character long, and dovecot is configured to use the + sign. In my case I decided to also allow the following: ".-" "__" and ".."

My last router in exim is mysql_user and the one before that is mysql_alias. I added the following before mysql_alias:

suffix_translate: debug_print = "R: suffix_translate for $local_part@$domain" driver = redirect domains = +virtual_domains local_part_suffix = .-* : __* : ..* data = ${quote_local_part:$local_part${sg{$local_part_suffix}{\N^(\.-|__|\.\.)\N}{+}}}@$domain # the following is an "optimization" or just a way to make the debug output less tedious. It prevents # exim from going all the way back to the first router with the new address redirect_router = mysql_alias

In the dovecot_lmtp transport, I added the rcpt_include_affixes option.

With LDA, use the -a flag as follows: -a $local_part$local_part_suffix@$domain

With LMTP, using the envelope_to_add option and configuring dovecot to use it with the lda_original_recipient_header option, I get an Envelope-To header populated with the original recipient, and dovecot uses that one for some reason. See my other message posted on this list.