Maximum size of sent/received e-mail including all attachments is approximately 21 MB.
On the server the maximum size is set to 25 MB. Maximum size of one attachment can be only 20 MB. E-mail clients (Thunderbird, Outlook) is transforming e-mails (or more precisely binary files) before sending with coding base64. File transformed in this way takes up about 33% more space then the original. Due to that true size of the e-mail has to be smaller by approx. one third of the maximum, therefore 20 MB.
Warning!
Most of the servers allows the maximum size of an e-mail to be about 6 MB. If you try to send larger e-mail, our servers will send it but destination server may not accept it. Therefore we recommend to send e-mails smaller than 6 MB.