I use Thunderbird and don't experience this nor am I seeing svchost.exe being involved in email sending/retrieval (generally I only see svchost.exe in relation to windows updates).
When concurrent connections are exceeded, avast can't actually scan prior to sending/receiving. Are you only experiencing this during sending ?
- Concurrent Connections Error: You could modify the EmailShield.ini file using notepad, C:\ProgramData\AVAST Software\Avast\EmailShield.ini (Vista, Win7 or later), this location may be hidden, depending on your windows explorer settings (Explorer > Tools > Folder Options > View)
Find the [EmailScanner] section and insert a new line after the section heading, and enter this MaxConnections=75 (if that line exists you can change the value to 75). Save the changes, the avast self-defence will ask if it is OK answer yes.
[EmailScanner]
MaxConnections=75
The default concurrent connection limit is 50 (for mail & news together). This limit exists not to save resources, but to prevent connection looping in strange configurations, so the user can increase it without worry to, for instance, 75. The value 0 disables the limit. That said your number of concurrent connections is under 50, so I don't know why you would be getting this error.