SugarCRM allows for multiple email addresses to be added to any Lead or Contact. Only one of the email addresses is designated as the 'Primary' email address.
Using Scribe Online's SugarCRM connector, the 'Primary' email address is retrieved and accompanies the other Lead and/or Contact fields you've mapped to Acoustic Campaign. You may choose a different Email address as the Primary. The SugarCRM record is modified and then synced.
If the email address is changed in in Acoustic Campaign, a sync to SugarCRM would cause the existing primary email address to be replaced.
The above statements assume you've not altered the set of Sugar Scribe Mappings that is provided by Acoustic Campaign.