If you want to send emails to your clients from ONLYOFFICE
portal, please, note, that ONLYOFFICE CRM
module allows you to send messages without opening any external email client software, all you need is to specify the parameters of the SMTP server for the mail service you commonly use. You will be able to send mails to your clients from your usual email address via the ONLYOFFICE
To learn more information, please, follow this link: http://helpcenter.onlyoffice.com/administratorguides/mass-mailing.aspx
If it doesn't answer your question, please, specify more detailes.