The Primary Consideration When Selecting A Tool To Perform The Conversion

        The presence of multiple email services and desktop email clients suggests that there is an even greater number of possibilities to migrate between a given pair of services/applications.  Obviously, the information (emails, attachments, contacts, calendar) is stored in a given format in every one of these apps/services. While some formats are highly standardized across applications, others are more flexible and vary greatly from one developer implementation to another.

Hence, it is of great importance to make sure that the software tool that is to be chosen is able to process the given format. The ideal solution is to choose an app that was specifically designed or tested to work with a given pair of email clients/services. For instance, if the intention is to export Thunderbird to Outlook, one must make sure the tool can correctly read MBOX files and can convert all the information, in its entirety, to the PST format.

Other Aspects To Consider

Some of the key aspects and features of email conversion software worth taking into account are as follows:

• full automation of the conversion procedure – for maximum convenience, the conversion tool should automatically locate the data storage files from the installation of the desktop email client. Although this can be done manually, in some cases, the names and structure of the folders are not intuitive at all, with different locations being used for storing contacts, message bodies, attachments, etc.;
• the tool should provide an intuitive, step by step conversion process, with a decluttered interface;  

• the ability to repair damaged files – some files will likely contain minor flaws, so it is a great bonus if the conversion tool can help fix these, enabling retrieval and conversion of such emails;
• the ability to correct the email encoding when transitioning between different operating systems (for instance, MAC OS and Linux encoding to Windows encoding when transitioning to Outlook);
• the possibility to extract the messages to one of the golden standards (.eml files) that can be imported by many other clients. You never know what email client you will be using in the more distant future. Thus, while you are migrating to Outlook, for instance, you might want to store the emails also in a standardized format, especially if the license for the conversion tool is valid for a limited period and you want to maximize its use;
• if security is a big concern (email data can often contain sensitive information), one key aspect is to make sure you are using a standalone application rather than uploading your data to an online platform (it can be hacked during the transmission or on the respective servers).

Considering all or most of these requirements will likely help to identify a well-rounded product that would ensure a smooth transition. Please note that with some online services, one would first need to export emails (for instance as .eml files) from the respective online mailing service, before being able to proceed with the next steps of conversion.