Reproduction steps:
1. Allow less secure apps to access your email account.
- Google guide: https://support.google.com/accounts/answer/6010255?hl=en
- Unity account may not be viable for security reasons (Tested with personal email).
2. Open the attached project.
3. Open EmailTestingScript.cs
4. Fill 'TODO' sections.
- Tested with smtp.gmail.com host and 587 port.
5. Play the Main scene.
- Success message will show.
6. Make sure Scripting Backend is set to IL2CPP and Scripting Runtime Version to .NET 4.6 in iOS Build Settings.
7. Build and run in iOS device.
-An email will not send with the following message:
...
System.Net.Mail.SmtpException: Message could not be sent. ---> System.IO.IOException: Connection closed
at System.Net.Mail.SmtpClient.Read () [0x00000] in <00000000000000000000000000000000>:0
at System.Net.Mail.SmtpClient.SendCommand (System.String command) [0x00000] in <00000000000000000000000000000000>:0
at System.Net.Mail.SmtpClient.SendCore (System.Net.Mail.MailMessage message) [0x00000] in <00000000000000000000000000000000>:0
at System.Net.Mail.SmtpClient.SendInternal (System.Net.Mail.MailMessage message) [0x00000] in <00000000000000000000000000000000>:0
at System.Net.Mail.SmtpClient.Send (System.Net.Mail.MailMessage message) [0x00000] in <00000000000000000000000000000000>:0
at EmailTestingScript.Start () [0x00000] in <00000000000000000000000000000000>:0
--- End of inner exception stack trace ---
at System.Net.Mail.SmtpClient.Send (System.Net.Mail.MailMessage message) [0x00000] in <00000000000000000000000000000000>:0
at EmailTestingScript.Start () [0x00000] in <00000000000000000000000000000000>:0
...

Reproduced with:
.Net 4.6
2017.1.1f1, 2017.3.0a7
iPad Mini 2 10.3.1, iPhone 6 Plus 8.0

Not reproducible with:
.Net 3.5
5.6.3p2 (feature was not introduced)